> When I upgraded Vivaldi-Snapshot to the most recent build, I suddenly lost
> all my settings: started getting cookies I didn't want, V would remember my
> browsing history, even reset my home page, totally undid my preferences.
Vivaldi creates a separate set of setting for its development build, just the same way Firefox 
creates spearate profiles for Nightly or ESR builds. Check out ~/.config/vivaldi and 
~/.config/vivaldi-snapshot

>  my settings were wiped after an upgrade.
In the past custom CSS and JS were being wiped out during upgrades. Now Vivaldi has improved and 
only custom JS can get wiped out - it definitely gets disabled, but with a bit of luck the 
custom.js file might still be in place after an upgrade.

> Vivaldi should have about:config.
+1 on that. Conversely, Firefox should have interface inspector to ease customization of user 
interface.

> Among other browsers out there, IMHO worth checking out: Midori, Qupzilla,
> and Seamonkey. There is supposed to be a Chromium-ungoogled, too, something
> like that, available in repositories: Chromium with everything Google
> removed. I just heard about it a few days ago.
Chromium is Chrome with everything Google removed. Or so the internet claims, because I haven't 
inspected the source code myself.

Janek