This appears to be related to Firefox's annoying tendency to auto-update itself, and Windows 7 64-bit's annoying tendency to require you to run any application in the midst of making changes to itself as an administrator (of course without telling you -- you get to figure this out on your own).

Every time I run into this situation, it is a 3-step process to solve it:

1. Open Task Manager and kill any still-running instances of firefox.exe
2. Open the Start menu, locate and right click on the Firefox icon and select Run as Administrator. (Note: If you have Firefox pinned to your taskbar, right clicking on the pinned icon will not give you the Run as Administrator option)
3. Watch Firefox go through the tail end of its update process (the one you never asked it to do, but which it decided to do anyway).

After this, you should be able to close Firefox, and resume running it normally, without the need to run as an administrator -- at least until the next time it decides to update itself again.

I have windows 7 & I'm able to right click on the firefox desktop icon and select 'run as administrator'.

Check that you do not run Firefox as Administrator.

Right-click the Firefox desktop shortcut and choose "Properties".

Make sure that all items are deselected in the "Compatibility" tab of the Properties window.

• Privilege Level: "Run this program as Administrator" should not be selected
• "Run this program in compatibility mode for:" should not be selected

Also check the Properties of the firefox.exe program in the Firefox program folder (C:\Program Files\Mozilla Firefox\).