This could b a problem with the file that tells Firefox what to do with what type of file.

Deleting mimeTypes.rdf will reset all download actions to the default settings. In Firefox 55+, the file name was changed to 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.

It's solved. Thanks you :)

Hello,

I am glad to hear that your problem has been resolved. If you haven't already, please select the answer that solves the problem. This will help other users with similar problems find the solution more easily.

Thank you for contacting Mozilla Support.