Switched to Vivaldi last year and haven't looked back. Did some side by side with FireFox for a month or two on my phone. I have a cheap 2022 Moto G something or other, running whatever Android it shipped with.

I guess that like a lot of people, I don't like having apps tracking stuff, but my work requires me to have access to Facebook, Insta, Threads, and the like... so, I just use browser shortcut widgets for them instead (I should quit my job, I know, I know... working on it). Both Firefox and Vivaldi immediately figured out that I wanted to run them in containers so that was great. However, Vivaldi runs all of them so smooth where as Firefox just kind of stumbles around. Some of them would refuse to work some days, just bringing up the web browser container and then crash. Facebook dot com was the worst... there were issues with the UI not showing me the text input bubbles and latency with button presses was terrible... like needing a refresh to show a "like" or even that a notification was read. It was almost unusable. Bizarrely, Outlook was also bad on FireFox... like that's a fairly bog standard email client and "productivity" site, but on FireFox it would crash more than it worked. Vivaldi handles all of the sites/platforms I need like I'm running the apps.

Maybe it's something with my cheap ass phone and Motorola's bloatware, but Firefox crashed and burned more than it worked. I cannot recommend Vivaldi enough.