I used Firefox from version 1.5 to version 30~32 alongside Opera (with the Presto engine), after that I switched to Palemoon since it had a pretty stable UI. A few versions later, before Quantum release, Palemoon felt really slow and webpages broke way too much for my taste. So I switched back to Firefox.
I got used to addons like Classic Theme Restorer only to be broken a few months later with the Quantum release and the addon apocalypse. I tried to stick with it for quite a while but it was quite tiresome to update CSS files every new release of the browser to keep the looks consistent.
Also, things like Google Docs/Spreadsheets, Office 365, JIRA really felt slow in Firefox... So I discovered Vivaldi, which has a lot of customization settings available and after giving it a few months trial I switched to it. And I've been quite happy with it, personally it's the best browser I ever used since good old Opera Presto, it's fast enough, clean and easy to customize. Heck, it even has CSS support and they manage to not break it with every new release.
And honestly, I don't see why I should go back as long as the first issue isn't addressed by significant leadership changes. Mozilla has repeatedly demonstrated their willingness to trample my privacy, why should I ever trust them again?