17-Feb-19: I don’t feel as secure about Vivaldi re. user privacy anymore (I’m using Iridium-browser these days as it looks to - arguably - be about the best of a bad lot). https://spyware.neocities.org/articles/vivaldi.html
1-Aug-18: Updated the Add-ons section with the addition of ScriptSafe.
Note: The Free Software Foundation recommends us to NOT use any Chromium or Chromium code based browsers due them phoning G**gle (often). The Vivaldi team say that they have plugged all known leaks to G**gle. The following two linked pages are well worth a reading:   My thanks to "kdemeoz" from the Manjaro forum. ;)
- 1 Introduction
- 2 Optimising & Customizing Vivaldi
- 3 Making Vivaldi the Default Browser in Manjaro
- 4 Some useful add-ons for Vivaldi
- 5 Support
Hard to believe I ever moved from Firefox to Pale Moon. Then after some weeks of using the Chromium code based Vivaldi, I happily made it my default browser (for a while, until I was put off by the leaks of its engine to G**gle - see the note at the top of the page & most especially the 2 links there for what looks like resolution to the leak problem).
Many are finding Vivaldi attractive due to its polished presentation; the incorporation of many options (most of which I'll never even discover as I don't need them); & (a necessity for me) the ability to use extensions (add-ons) from the Chrome Web Store 
Below I'll add some things that a new Vivaldi user, or someone who is thinking about using Vivaldi may find helpful. This page will grow as I learn more, &/or others come & contribute to the page.
Optimising & Customizing Vivaldi
This is a short section that may be of particular use to those who are running Vivaldi on not so powerful machines. I'll post this link  to a page on www.ghaks.net that has been written for the Win10 OS. The important stuff is very easily translated to any OS that is running Vivaldi.
Vivaldi Tips  is a great site for a new Vivaldi user, as it has a number of user added tips laid out in an easy to read format. Well worth a read.
Making Vivaldi the Default Browser in Manjaro
You'll find a good page here  on the problems that users of Linux distros face due to the lack of a default way that is used by the KDE; Gnome; & other package makers for setting & sourcing the default browser.
I use Openbox, on a Manjaro system that I created for myself in the Arch style (though a bit quicker) using the Manjaro Net-install. As can be gathered from the above section, I have long been a Firefox user, who then moved to Pale Moon, using it for quite a number of months. Pale Moon was set as my default browser until I cancelled that setting in Pale Moon some days ago.
How to check if there is a default browser
A way to check which is your default browser is to type env into your terminal. This will display a list of all of your user environment variables. If you type sudo env you'll get a shorter list of root specific env' variables.
When I typed env (even though I'd turned off Pale Moon's default browser weeks ago) listed in the terminal was BROWSER=/usr/bin/palemoon . So I went looking & found the /etc/environment file in which I found BROWSER=/usr/bin/palemoon . So I changed the palemoon to vivaldi-stable & tested with a fresh terminal. It still said palemoon. So I rebooted & it still said palemoon.
After that I had a look in my ~/.bashrc & discovered another BROWSER=/usr/bin/palemoon line, so I changed that to vivaldi-stable & checked in a fresh terminal & now my BROWSER environment variable was set.
To check it I typed the following into the terminal (note: the $ is required, & has no space between it & BROWSER):
& that created a new tab in Vivaldi which then loaded up the Manjaro forum's main page.
At this point that is good enough for me.
To sum up
Here are a few things to finish this subject off:
Firstly, if you enter sudo env at the terminal prompt you'll get a shorter list of root owned environment variables, which in my case & it should be yours too, also included BROWSER=/usr/bin/vivaldi-stable . It is a very bad idea to run any browser as root & surf the web. Don't do it unless you know exactly what you are doing & why.
Secondly, if you don't know which type of Vivaldi build you are using, have a look for it in your /usr/bin/ directory. I'm obviously using the /usr/bin/vivaldi-stable install. There also exist vivaldi-snapshot & vivaldi-beta , there may be others that I'm unaware of too. So just be sure to use the right vivaldi-<name> when setting up your default $BROWSER environment variable.
Thirdly, there are other ways that may work for you to make Vivaldi (or any other browser) the default. Most browsers seem to have an inbuilt way that works. Vivaldi's inbuilt way does not work with Linux at least. There are other commands that can be entered into the terminal. I tried a couple & they did not work. (If you haven't already, have a read of this page  that linked to earlier on, I found it to be very educational.)
My suggestion is to check if the file /etc/environment exists. If it does not, I'd initially at least, not worry about creating it, though it likely is useful if you wanted to use Vivaldi as root , which as I've already said is a very bad idea.
Then open ~/.bashrc in your favorite text editor & just add the following line BROWSER=/usr/bin/vivaldi-stable (remembering to change vivaldi-stable to whichever Vivaldi package is the one that you installed - as mentioned above).
The good news about setting the environment variable in your ~/.bashrc is that it is read last & is therefore the one that matters most.
Call your default browser from the terminal
After that, open a new terminal (or close & reopen - or otherwise refresh your terminal) & enter
at the terminal prompt. If Vivaldi is running you should get a new tab with the Manjaro forum in it. If Vivaldi is closed, it will open up & give you that new tab.
This is also another way to test if you have a default browser & which one it is.
Some useful add-ons for Vivaldi
Without access to what I consider to be some essential extensions or add-ons for Vivaldi then no matter how good the browser is, I would not use it. I consider personal privacy to be a right that every individual born on the planet is entitled to. Unfortunately that is far from the way that governments of all shapes, sizes & countries see the situation. Then there are every shape & size of those that belong to the marketroid species out there that want to know absolutely everything they possibly can about you. Whether they use that information themselves, or just sell it on to those that they collect for.
Nobody asked for my permission to follow my every move on the internet (or anywhere else for that matter). I don't want to be part & parcel of any package of people, or any kind of demographic that is processed by extremely sophisticated algorithms so that others can profit from the knowledge one way or another.
So to that end, I like to be able to make it more difficult & therefore more costly for the powers that track to do so. Apart from having more pleasant, less cluttered & distracting web pages to look at due to the functions of add blockers. Removing that rubbish also makes my pages load that bit faster too.
Add-ons to help protect your personal privacy
Here is a list of add-ons that I use, most of these are also available on Firefox & Pale Moon. These guys all work very well together & from my experience are reliable & have no negative effect on Vivaldi's stability. (Some of these add-ons I'm still investigating & I expect that I'll remove one or two of them & also likely reconfigure the likes of uMatrix & uBlock Origin to work more smoothly with Disconnect - I've turned off ShareMeNot as Disconnect looks to have that covered).
All of these add-ons are available through the Vivaldi Tools/Extensions Menu which will get you to the Chrome Web Store  where you can search out add-ons & start investigating any that interest you:
Note: if you use ScriptSafe (see below) then you won't need to use Canvas Defender - or any other canvas fingerprinting protection tool.
Canvas Defender - Instead of blocking JS-API, Canvas Defender creates a unique and persistent noise that hides your real canvas fingerprint.
Disconnect - Make the web faster, more private, and more secure.
HTTPS Everywhere - Encrypt the Web! Automatically use HTTPS security on many sites.
Privacy Badger - Privacy Badger protects you from trackers as you surf the web!
ShareMeNot - Prevents third-party buttons embedded by sites from tracking you until you actually click on them.
uBlock Origin - Finally, an efficient blocker. Easy on CPU and memory.
uBlock Origin Extra - A companion extension to uBlock Origin: to gain ability to foil early hostile anti-user mechanisms working around content blockers.
uMatrix - Point & click to forbid/allow any class of requests made by your browser. Use it to block scripts, iframes, ads, facebook, etc.
ScriptSafe - A great alternative to uMatrix. In fact, from my tests, it does all & more that uMatrix does. So ScriptSafe has become my preferred choice. Well worth a look.
I'd prefer, instead of uMatrix or ScriptSafe to use NoScript in combination with RequestPolicy, unfortunately they are not available for Vivaldi.
Self Destroying Cookies - This one seems to have arrived since last I was looking into Vivaldi & it is the closest thing I can get to the Firefox add-on Self-Destructing Cookies. It is great, & at last I'm satisfied re. having an add-on that deletes all cookies (not in its white list) as soon as all tabs connected to the same domain are closed. BRAVO! This add-on is very easy to use too.
Flash Super-Cookies - The persistent flash cookies are placed in ~/.config/vivaldi/Default/Pepper Data/Shockwave Flash
You can make an alias so that you can quickly manually delete them:
alias rmvflash='rm -R -f ~/.config/vivaldi/Default/Pepper?Data/Shockwave?Flash'
You can of course make the alias name "rmvflash" to be whatever suits you.
Personally, I have no need for Flash, but many people still must use it.
Using Firejail to sandbox Vivaldi
Firejail is not a Vivaldi extension/add-on. What it is, is an extremely valuable security feature that can be used with not only Vivaldi. So I have added it in here below the privacy/security add-on section.
Firejail  is a very easy to use piece of software, initially developed to make Firefox more secure by isolating it (putting it in a sandbox) from the rest of your system. Firejail has developed beyond that & can be used simultaneously on many parts of your system. It is worth reading about on Firejail site (linked to above).
Beyond that, I can say that Firejail & Vivaldi work seamlessly. You need to start Vivaldi-stable like so: firejail vivaldi-stable Which is easy in any DE/WM that allows you to enter your own menu commands (I use Openbox where it is simple). Or, you can make a ~/.bashrc alias to call Vivaldi from the command line.
Add-ons that enhance Vivaldi
Here are some add-ons recommended by some of Manjaro's Vivaldi users:
Auto Page Refresh - Auto Refresh and reload pages on a selected interval.Sound Feature included.
OneTab - Save up to 95% memory and reduce tab clutter.
PDF Viewer - Uses HTML5 to display PDF files directly in the browser.
I'll include other add-ons that people on the forum recommend.
The Great Suspender - Automatically suspends unused tabs to free up system resources.
Following is a link to this page's forum counterpart where you can post any related feedback: