This is due to wrong content type set for these files (a server issue). Your server sends these files with application/pdf content type. You can fix it with this add-on: Content-type fixer.

Thank you, but please go back and find the real issue. ALL other browsers (Chrome, Safari...) download the Illustrator files with no issue. They (download) the .ai extension without adding any (Content-type fixer) plugin or other add-on or altering the extension (to pdf). Firefox had NO issue prior to the last few updates. What changed? I like Firefox but now use Chrome.

Brian Sauriol

Please read /questions/1436707. Someone with Firefox 121 asked the same question. So nothing has changed here during the last few updates.

It may be irritating to you, Brian, but this is a proper way of downloading files and every other browser is doing it wrong.

Thank you for your response. I understand...although I downloaded the 'Content-type fixer' extension (as per your first reply) and activated it in Firefox Settings > Extensions & Themes. But the Illustrator Files still download as .pdf. No clue why. I have to manually type in the '.ai' extension EVERY time. I don't want to use the Info Window and change the 'Open With' settings because then all my pdf files will open in Illustrator. Yes..this is frustrating...