I'm not sure if this is a bug or a "feature," but it's definitely annoying.
If you upgraded to Firefox 4 last week, you may have noticed that the status bar -- which formerly resided at the bottom of the browser -- is gone. Well, not so much gone as hidden: a pop-up status bar now appears in the lower-left corner whenever you mouse over or click a link.
Except when it doesn't. The other day I noticed that the status bar seemed to have vanished. Turns out it had just relocated -- to the lower-right corner of the screen! The next day, it was back on the left side. Huh?
It took me a few days to figure out what was going on here: whenever the Find bar is visible, the status bar jumps to the right side of the screen. And I use Find all the time, which is why this happened so often.
What's the fix? To my knowledge, there isn't one yet -- other than to close Find (by clicking the little 'x' at the far left) when you're done with it. Hopefully Mozilla will fix this "feature" with the next Firefox update.