There's two things that have really been annoying me about Firefox lately.
First, sometimes, when Firefox pops up a "modal" dialog (e.g. a confirmation dialog that's supposed to remain on top and disallow you from interacting with the application), if Firefox isn't the current application (i.e., is behind another window, e.g. Word), the dialog shows, but can't actually be found. When you switch back to Firefox, it's, of course, waiting for some input to that dialog. So you can see your Firefox window, but you can't actually interact with it. You can't even click the Close Window ("X") button. So the only way to close it is to kill it! I'll hazard a guess and say the modal window exists *somewhere* -- perhaps, hidden in Firefox itself behind the main window. There's no consistent pattern...
The other thing that's really been shitting me is in recent versions (this bug didn't seem to be present in the 1.0 series -- only 1.5+), is your browsing history sometimes just mysteriously disappears. The entire history -- in the History sidebar, in the address bar dropdown -- all gone. As above, there doesn't seem to be any explaination for this, and there's no consistent pattern.
Now I'll get the Open Source retards hounding me telling me to log a bug. No freaking way. Every time I log a bug, I get flamed. Usually because the bug is "unreproducable". For examples, look at the PPP bugs in Debian.