- Unplug the second monitor.
- Start Firefox.
- Plug in the second monitor.
- Move your Firefox window to the second monitor.
- Type an m (or any other character -- I chose m because it will auto-complete to macosxhints.com) in the URL bar.
This annoying behavior doesn't go away until you restart Firefox, which results in a loss of all of your open windows. This morning, I came up with a workaround to make this restart a little less annoying. Instead of quitting Firefox normally, Option-Control-click on Firefox in the Dock and choose Force Quit from the pop-up menu. Then restart Firefox. Firefox will believe that it crashed, and give you the option to restore your session with all of the windows and tabs you had open previously.
[robg adds: I think a more elegant solution would be to install one of the many available session saving extensions for Firefox. With something like Session Manager or SessionSaver .2 installed, you can have Firefox automatically reopen all windows and tabs that existed when you last quit the program. Note, however, that I haven't tested these solutions with the specific two-monitor problem listed here, but I think they should work fine.]

