User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:73.0) Gecko/20100101 Firefox/73.0

Steps to reproduce:

In Options-> General -> Application -> pdf I set it to "Always ask".
When click on an pdf link an window opens and asks what to do.
Default is "open in Firefox", I click OK.

Actual results:

A new Tab opens and Firefox opens a window and asks what to do.
Default is "open in Firefox", I click OK.

A new Tab opens and Firefox opens a window and asks what to do, and so on

Expected results:

I would expect it to show the pdf after after clicking on "open in Firefox".

Previously I got the recursion without any user interaction.
It was constantly opening new tabs.

Sadly I cannot reproduce that behavior after a change to the action type associated with pdf.

Did you associate Firefox with PDFs in Windows? If so, this is a duplicate of bug 215554.
If not, then at the "What should Firefox do with this file?" dialog you chose to open with Firefox, and this is a duplicate of bug 218257.

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. You can get around that with Bypass Forced Download or Open in Browser. Less commonly, servers can also fail to correctly identify the file type by sending Content-Type: application/octet-stream. Extensions like Header Editor can override that.

Yes seems to be a duplicate of 218257.
Sad, that this bug seems impossible to fix.

(In reply to Gingerbread Man from comment #2)

at the "What should Firefox do with this file?" dialog you chose to open with Firefox, and this is a duplicate of bug 218257.

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. You can get around that with Bypass Forced Download or Open in Browser. Less commonly, servers can also fail to correctly identify the file type by sending Content-Type: application/octet-stream. Extensions like Header Editor can override that.

(In reply to leifeld from comment #3)

Yes seems to be a duplicate of 218257.
Sad, that this bug seems impossible to fix.

*** This bug has been marked as a duplicate of bug 218257 ***

This Bug is possible to fix, it's just some people lack a desire or knowledge to do so.
For Users Step-By-Step: 1) Switch app preference for pdf from Preview to Ask, 2) Try to open PDF and then click CANCEL, 3) Go back to app preference and switch back to Preview.
THIS way you can fix it making it work by default. Also, never again choose Open via FF in that dialog window... or until Devs fix this bug.
For Developes of Firefox: you need to offer an additional option of 'Preview' when Dialog window asks what to do with pdf: Open with FF, Preview in FF, Other.
Alternatively, at least make sure that choosing in about;preferences PDF to Preview would fix it in one click instead of that three-Steps way I wrote above.