This was my experience...
Hmm, a WebGL application I'm developing isn't receiving my right-click gestures. I guess that's the mouse gesture functionality I remember from Opera. No worries, I'll just change the bindings...
I opened the settings and found the mouse gesture settings easily. Hmm, okay there's this bind to Alt checkbox, I'll enable that.
Back in my WebGL app... Hrm, still not working. On closer inspection I see the Alt binding only works for trackpads. Okay, fair enough... I guess. I'll just change from right-click to middle mouse, or something... Wait, no options for that. Okay, fine I'll just disable the darn thing.
Now my WebGL app works great. Yay!
Hmmm, but I feel like I'm not getting the proper Vivaldi experience without those gestures. I know! Vivaldi is open source, I'll just implement some configuration options myself...
Wait, they only offer tarballs? How the heck do I submit pull requests? Presumably I can't. Alright, I’ll email them asking if they plan to accept pull requests any time in the near future.
Hmm, their contact form keeps failing with cryptic error messages, but never explaining what I did wrong.
Alright, I’ll go to their bug tracker. Okay, I need to sign up. Wait... what? Their bug tracker registration form is also failing but not explaining why.
Okay, to be fair, by this point I'm guessing it's because I'm using an addition (plus) character in my email address. Not supporting that is poor, but not informing me that the email address field is the issue is totally unacceptable.
Alright, I'm far too annoyed at this point. If Vivaldi can't manage an open source project, can't write a working contact form and can't write a working sign-up form then I have very little evidence to suggest they can write a decent browser.
Firefox it is.