User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:65.0) Gecko/20100101 Firefox/65.0

Steps to reproduce:

I opened a pdf file that i downloaded via Firefox

Actual results:

After opening it, Firefox started opening the same file(in a new tab each time) non stop until i closed the browser and then it reopened itself and started doing the same thing. Upon restarting it manually again, the issue got resolved.

Expected results:

Opened the pdf file in just one new tab

Actually after restarting my computer, the bug still occurs constantly when opening a pdf and i don't know how to fix it.

The web doesn't work with file extensions but rather with content or mime-types.
You very likely assigned a content-type that Firefox doesn't support to be opened with Firefox.exe.
Is that correct ?

(In reply to steliospapamichael17 from comment #1)

i don't know how to fix it.

1. Enter about:support into the location bar.
2. Click the Open Folder button. This opens a file manager window with your Firefox profile folder.
3. Exit Firefox.
4. Back in the file manager window, delete the handlers.json file.

If you get a "What should Firefox do with this file?" dialog and you choose to open with Firefox, that will create an infinite loop. That dialog is for external applications only, and you're not supposed to be able to pick Firefox there [1]. To do that safely, use the Open in Browser extension [2].

The only way to view PDFs in Firefox is to set the action to "Preview in Firefox" in about:preferences. However, servers can still trigger the "What should Firefox do with this file?" dialog, usually by sending the Content-Disposition: attachment header. One way to get around that is the Bypass Forced Download extension [3]. Less commonly, they can also fail to correctly identify the file type by sending Content-Type: application/octet-stream. Extensions like Header Editor can override that [4].

[1] Bug 218257
[2] https://addons.mozilla.org/firefox/addon/open-in-browser/
[3] https://addons.mozilla.org/firefox/addon/bypass-forced-download/
[4] https://addons.mozilla.org/firefox/addon/header-editor/

(In reply to Matthias Versen [:Matti] from comment #2)

The web doesn't work with file extensions but rather with content or mime-types.
You very likely assigned a content-type that Firefox doesn't support to be opened with Firefox.exe.
Is that correct ?

That's correct. I didn't know Firefox doesn't support that. Thank you.