Well, it looks like you have a new version of Firefox, so you must have upgraded that recently. And if you were one of the millions who got whatever security patches or upgrades for the recent virus and worm attacks, that could also be interferring with something.
If it only happens when working on WordPress, then the issue may be the browser and not WordPress. And if no one else comments here or on the forum with a similar story, then the odds are that it is something conflicted on your system.
Before the SP2 patch on WinXP, some three key combination would lock up my entire computer. I never found the combination (only the left hand) and it did the same thing with three different programs, so it was really hard to narrow it down, but it seemed to fix itself in time or I stopped doing whatever that three key combination ways.
Computers - not a perfect science.