It's possible that another running program or a previous instance of Firefox that did not close properly is interfering with the Firefox update process. This error can also occur if you share your computer with other users, and another user's account is still logged in with Firefox running. The easiest way to resolve such problems is to restart your computer. Once the computer starts up again:
Click the menu button
and click help
.On the menu bar, click the Firefox menu.
Select About Firefox. Firefox will begin checking for updates.
Close other running programs
Some programs that can interfere with Firefox are set to start up automatically when you start your computer. Before updating Firefox, try closing or disabling such programs; for example, Spybot TeaTimer and Logitech QuickCam software have been known to interfere with Firefox updates.
Run as Administrator
If you are using Windows 7 or Vista:
Restart the computer so that no Firefox processes are active.
Open the Windows Start Menu, type firefox and right-click on the "Mozilla Firefox" entry that appears.
Select Run as Administrator and Continue if the User Account Control dialog comes up.
If the issue is not resolved, close Firefox and restart the computer again but this time, open the program folder where Firefox is installed (e.g., open the C:\Program Files\Mozilla Firefox folder or, on 64-bit windows, the C:\Program Files (x86)\Mozilla Firefox folder), right-click on the firefox or firefox.exe file icon and select Run as Administrator.