Recently there is a Mozilla software update related problem which cause this alert:
Warning
One or more files could not be updated. Please make sure that all other applications are closed and that you have permission to modify files, and then restart Firefox to try again
In order to fix it do the following:
1) When the dialog box says "One or more files cannot be updated..." use "Task Manager" to kill the application (firefox.exe or thunderbird.exe)
2) Locate "C:\Program Files\Mozilla Firefox\updates\0" and delete the contents of this folder.
3) Now, you can start Firefox as normal.
4) Go to tools>extensions and disable all extensions (don't uninstall, this is not necessary)
5) Restart Firefox and go to help>check for updates
Note
If it says "apply updates" (or a similar text) in the help menu, do the following: go to C:\Program Files\Mozilla Firefox and locate active-update.xml. Open this file with notepad. Delete the content of this file and save it, now proceed with step 5
6) The update will be downloaded and applied. Enable all your extensions when the program is updated.
The problem occurs when some extensions keep their files "read only". By disabling the extensions temporarily, the update can be applied.
Note
If aboved not was not help to you and you work on restricted user accound. Download the latest Mozilla software and install it as administrator over the previous installation
Tip
If application doesn't start see in task manager after a process called xpicleanup.exe and kill it. Next delete the xpicleanup.dat form installtion folder. Restart the app, and it should be working by now