Deleting '''mimeTypes.rdf''' will reset all download actions to the default settings. In Firefox 55+, there is a new file named '''handlers.json''' Type '''about:support'''<enter> in the address bar. Under the page logo on the left side, you will see '''Application Basics. ''' Under this find '''Profile Folder. ''' To its right press the button '''Show Folder. ''' This will open your file browser to the current Firefox profile. Now '''Close Firefox. ''' Locate the above file. Then rename or delete it. Restart Firefox.