Hello Christine, look here: https://support.mozilla.org/de/questions/1432666#answer-1643746 i had the same problem and this was the solution.

Hi Christine, this problem occurs when the updater wasn't completely successful in replaced all the older version program files. Maybe a file was locked? If you download and run the full installer, it should complete the job. In fact, it should recognize that you already have Firefox and do a seamless update. If you aren't confident, you could consider making a profile backup. * Download page: https://www.mozilla.org/en-US/firefox/all/#product-desktop-release * [[Back up and restore information in Firefox profiles]]