The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information. Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

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Try to delete the mimeTypes.rdf file in the Firefox Profile Folder to reset all file actions.

• http://kb.mozillazine.org/mimeTypes.rdf
• http://kb.mozillazine.org/File_types_and_download_actions#Resetting_download_actions

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder

@Tylerdowner, I'm sorry I couldn't accept your answer as I am hoping not to lose any extensions and information that I've already set on my copy of Firefox. @cor-el, will this cause the same result, i.e., delete information and remove extensions I've setup on Firefox?

No, a "Reset" creates a new profile and that causes the dataloss that you want to avoid.
You should normally only do a reset if you can't fix the current profile and it looks that the problem isn't caused by one of the files or data that Firefox recovers, so you won't move the problem over to the new profile (e.g. corrupted cookies).

If you delete one or more files in the Firefox Profile Folder then you only lose the data that is stored in the files that you delete.
In case of mimeTypes.rdf you reset all actions that you see in the Tools > Options > Applications window.
So if you can't fix it in that window then you can try to move or delete the mimeTypes.rdf file.