Currently on FF33.0.2 I used to open gif and swf files(also doesn't work on saved html files) on my local hard drive with Firefox just fine before by just manually double clicking the file and FF would automatically run it just fine.

Now it only opens up a new window in the startup page. Any other browser(Chrome, IE) that I use to open them works. Also, I can manually force FF to run the files by dragging or using the [Open] command in the menu bar but then it's much more convenient if it runs them automatically by clicking the files themself.