Deleting the '''mimeTypes.rdf''' will reset all download actions to the default settings. Type '''about:support''' in the address bar and press enter. 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.

See this: https://support.mozilla.org/en-US/kb/firefox-repeatedly-opens-empty-tabs-or-windows