Ideas that came up: - Disable your internect connection (e.g. by unplugging the network cable), start up Firefox and wait for the downloads to time out, after which you might be able to clean up the list of downloads if this happens within a reasonable time frame. This way you won’t need to change any settings in your computer OR - Start Firefox in Safe Mode (by pressing '''option''' (on Mac, '''Shift''' on Windows) while clicking the startup icon), or choose ''Restart with Add-ons disabled'' in the Help menu. When opening the Add-ons section, you will likely be able to remove the add-on responsible for this, and when restarting Firefox as usual, the downloads should probably have stopped. There may be more radical ways on file level (i.e. remove some Active Downloads file for the extension if you know where it resides), but personally I’d use the second method. For reference, could you name the add-on involved?