User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.113 Safari/537.36

Steps to reproduce:

1. Open Main menu -> Options -> Files and Applications ->Applications
2. Select "Always ask" option for Portable Document Format (PDF).
3. Open any site where you can open PDF files (I used a menu in a restaurant).
4. Click on file.
5. Popup is displayed in which you select how you want to open file.
6. Select Open with "Firefox".
7. Select "Do this automatically for files like this from now on" checkbox.
8. Click "OK"

Actual results:

A lot of empty tabs are opened one by one. Process may be stopped by closing the browser.
Main menu -> Options -> Files and Applications ->Applications -> Portable Document Format (PDF) contains "Use Microsoft Edge (default)" after browser reopening.

Expected results:

Only one document should be opened in separate tab in Firefox browser. Behavior of opening PDF files should be saved in preferences (Main menu -> Options -> Files and Applications ->Applications -> Portable Document Format (PDF) should contain "Preview with Firefox" option selected). Each time a user try to open PDF - new tab with document is opened automatically

Thank you for reporting this.

This is a duplicate of bug 1328864, which was closed as invalid by a contributor, but I don't think Firefox should let itself get into a trap like that (it does it with other URL handlers as well.)

I'm going to reopen the original bug, cc you on it, and refer it to the team dealing with URL handlers.

This is now being worked on in bug 1633790. So updating the resolution of this.