Sorry, this can be caused by a problem in the update. Could you download a fresh installer for your version of Firefox and run it again: * For most people, Firefox 135: https://www.mozilla.org/firefox/all/desktop-release/ * Only if you previously selected it, Extended Support Release of Firefox 128: https://www.mozilla.org/firefox/all/desktop-esr/ You do not need to remove your existing Firefox installation, the installer should detect and upgrade it. If you end up with the same problem, it's possible that a file is locked, either by another program (such as security software) or Windows. You may need to do a regular shutdown and restart of Windows and run the installer again in that case.