Normally it happens once or twice a day, it's fairly random but there does seem to be several things which will trigger it.
Sometimes it will happen when exiting Firefox, right-clicking on the desktop or Recycle Bin, browsing Control Panel, and generally browsing in Windows Explorer, usually whilst browsing my external USB drives where my music and video files are stored.
I've noticed it happens more during the evening but I think that is simply when the computer is used most.
Oddly it does not appear to affect the Guest account, however when I have tried creating a second account for myself, the problem persists.
I am fairly disgusted at the response from Mozilla, I linked to this thread in my bug report and the guy had quite obviously not even taken the time to have a look.
Here's several things I have done whilst trying to fix the issue:
Two clean installs, both from seperate downloads of Windows 7 RTM.
Removed MSE.
Removed Spybot - Search & Destroy.
Removed WinRAR.
Removed my Creative X-Fi Xtreme Audio card and
drivers.
Uninstalled and reinstalled Firefox.
Created a new profile in Firefox.
Created a new clean user account.
Removed EVERY entry using ShellExView bar the Microsoft entries.
Cleaned the Registry with CCleaner.
Removed CCleaner.
That's all I can think of for now but there's certainly a lot more things I have tried.
Someone mentioned that the issue does not occur under Windows 7 Professional, however I use BitLocker so it isn't really an option.
Microsoft say it's not Windows 7, and Mozilla claim (well that one guy does anyway) it's not Firefox.
I really am ready to throw the towel in.