I have Firefox 18.0.2 on Windows 7, but I think that this was happening in the previous version as well.
If I have the browser open, normally I have one Firefox window with 15 tabs ("primary") or so and another Firefox window behind it with 5 or so tabs ("secondary"). While it's a lot of tabs to have open, this is normally not a problem, and I don't mind if the browser is "busy" from time to time when things are autorefreshing.
The problem comes when, probably about 40% of the time when it's in a state of "not responding", the primary Firefox window completely disappears and I'm stuck looking at the secondary window behind it. If I'm already in the secondary window, I get the desktop or whatever other program is open. I don't mind waiting for the browser to come back from a not responding state, but I do mind that the window up and disappears in the meantime. Going to the taskbar and clicking on the Firefox icon and selecting the primary window restores it back to normal.
Is this a quirk of Firefox, a figment of my imagination, or is this meant to be a "feature"? Is there any way to stop the program from doing this?