Firefox nightly is pretty darn stable, though I'm not sure if there's any security issues with it. I use it for both mobile and desktop, and have for the past 5~10 years and haven't had any issues that weren't fixed by immediately downloading a fresh build (this happened twice) or were caused by me performing unsupported/insane actions (transferring browser profiles (indexeddb) on mobile via CLI commands using remote devtools).

I've got one buggy device, but I suspect corruption due to the error message and I'm more interested in saving the unsyncable data I've got on that profile than I am in trying to fix it by clearing data.