I actually switched away from Firefox to Vivaldi a few months ago mainly for 2 reasons.

Firefox's profiles are dogshit. They are almost a hidden feature and are very cumbersome to use.
The Android browser support for certain types of extensions is dodgy. Using uBlock Origin delays the loading of all webpages by a few seconds for some reason. There is a Github issue about this that has been open for a few years now.

I want the browser I use to be on both Android and desktop. Vivaldi has been OK so far.

I do miss a lot of the good stuff from Firefox, especially their address bar. For some reason I find it much better than anything on Chromium based browsers. Firefox's is much snappier and is correct with it's suggestions the majority of the time.

I also like Firefox's sync between devices to be much better.

When those 2 issues are resolved I will come back, but as it stands now it's a hassle for my needs.