I use FF on Mac OS X. It's been steadily becoming one of the worst browsers for the platform, performance-wise, but certain plugins still cause me to use it as my primary browser.
FF has always been shaky about remembering which monitor it should be on, but if I kept it there once I got it to open on the right monitor, it would at least always open where I left it. Well, in FF12, they have added this fantastic feature where all new windows open up on the primary monitor. Hooray! This should really increase my productivity. It might seem minor, but it's not minor to me.
Alright, that's annoying, but I decided to upgrade on day 0 of the release. My mistake assuming that they would stick with random interface changes, and not break lower-level functionality. I'll just roll back the browser. Fired up Time Machine, and I rolled back to the previous installation. Now, FF randomly hangs on various pages for up to a minute. Maybe the profile is hosed? Rolled that back, too. No, still hangs. Also tried starting in "safe" mode - it still hangs.
So, this isn't necessarily FF fault, maybe the rollback was corrupted. I'll just download FF11 and reinstall it. Except, since it's no longer the latest-and-greatest, it's not available. I couldn't find it without manually editing the FF12 link to point to 11.0.
Firefox, I don't know how much longer I can bother dragging your sorry carcass around with me. Your 3D transforms are so slow they are often unusable and the rapid update cycle is starting to cause real issues. Of course, I can't forget the random interface changes like removing favicons from the URL bar, because the interface is so terrible you can't tell the favicon from a security marker. I've now got almost as many interface hacks (via Stylish and plugins) as I do normal plugins.
I don't know what the solution is for FF, but I keep getting my hopes up, and keep getting more frustrated.
Note: I know that this might be only my computer. I don't have a lot of time or energy to set up another multi-monitor system, upgrade FF to FF12, and try it out. Since FF is one of the only applications I use that has multimon issues (besides a few random utilities), I have to assume it's something wrong with FF.