Never.
Firefox
A community for discussion about Mozilla Firefox.
I don't recall ever having to downgrade Firefox due to some bug, and I've been using it since the beginning.
Never happened in my life. Personally, if something breaks I just wait for it to get fixed.
I just checked and I have never downgraded Firefox on my current install. Interestingly I have downgraded Chromium 4 times.
I never have, but I typically run what's packaged by Debian, which packages Firefox ESR.
I'd imagine that if one runs nightly builds or something, one would hit some level of regressions.
I run nightly, not exclusively but pretty close, and I must say Firefox' nightly builds are pretty damn stable in my experience. For sure, there are situations where some feature y is clearly unfinished, but it's super rare to face a situation where I would even need to think about working around some issue - such game breaking issues just don't happen too much at all. Usually, if a build is found to be truly broken (like crashes very often etc.) then nightly updates get paused. I can remember maybe two times that I've had to revert to previous build in over ten years because the I had received an update before updates were halted.