https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

You can check the handlers.json file in the profile folder to see for what MIME type you created the allow exception.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/Profiles

> You can check the handlers.json file in the profile folder to see for what MIME type you created the allow exception.

I looked at about:support and about:profiles and about:config and there was nothing there like that. Must be somewhere else.

The profile folder: C:\Users\19.09.20\AppData\Roaming\Mozilla\Firefox\Profiles\hboxjgox.default-release-2

Does this happen with all PDFs from all sites, or is there a particular site or two with this problem?

If this happens on particular sites:

You can install my extension, enable logging by clicking the Zzzz button, and then see whether the server is sending a nonstandard Content-Type header, i.e., not application/pdf:

https://addons.mozilla.org/firefox/addon/content-type-fixer/

(If the file name has a .pdf extension, the add-on will switch the Content-Type to application/pdf automatically while it is enabled.)

It happens on all sites.

Try looking for the handlers.json file again. Perhaps your is corrupted and needs to be regenerated. It's a file in your profile folder.

Found it. This is what it says. I'm unfamiliar with this terminology. Sure hope someone here knows what it means.

{"defaultHandlersVersion":{"en-US":4},"mimeTypes":{"application/pdf":{"action":4,"extensions":["pdf"]},"application/download":{"action":4,"extensions":["pdf"]}},"schemes":{"ircs":{"action":2,"ask":true,"handlers":[null,{"name":"Mibbit","uriTemplate":"https://www.mibbit.com/?url=%s"}]},"mailto":{"action":2,"handlers":[{"name":"operamail.exe","path":"D:\\PROGRAM FILES\\EMAIL\\OPERA MAIL USB\\operamail.exe"},{"name":"Yahoo! Mail","uriTemplate":"https://compose.mail.yahoo.com/?To=%s"},{"name":"Gmail","uriTemplate":"https://mail.google.com/mail/?extsrc=mailto&url=%s"}]},"irc":{"action":2,"ask":true,"handlers":[null,{"name":"Mibbit","uriTemplate":"https://www.mibbit.com/?url=%s"}]},"magnet":{"action":4}}}

In relevant part:

"mimeTypes":{
  "application/pdf":{
    "action":4,
    "extensions":["pdf"]
  },
  "application/download":{
    "action":4,
    "extensions":["pdf"]
  }
}

Your Firefox has stored instructions for .pdf files served with two different Content-Type headers: application/pdf and application/download (the second may be used by sites that want to bypass viewing PDFs in a tab).

The action "4" indicates "use system default." So in theory Firefox will open PDFs using your system's default application for opening PDFs.

> So in theory Firefox will open PDFs using your system's default application for opening PDFs.

It seems this browser like to disrupts theories. I'm curious why, in Options, Applications, there is PDF Use Foxit Reader Portable Document Format (PDF) Preview in Firefox

Why 2 of them? Aren't they both the same? Now I notice that if I click the downward arrow on PDF (1st one) and choose "Use other" and select Firefox (since that is the fastest and seems to work just fine), it doesn't store. It just ignores what I chose. Is it rejecting itself? I can pick Chrome and it stores. I can pick Foxit and it stores. When I remove both Chrome and Foxit it still won't let me pick Firefox! Is this really normal? Why would it offer Firefox if it can't take Firefox? This is baffling. I even used Browse to navigate to Firefox in Program Files and it didn't work. Yet anything else seems to. There must be something corrupted somewhere that is blocking Firefox. Yet most everything else in the browser seems to be just great.

Also in the 2nd one it offers "Preview in Firefox" but that is not offered in the 1st one. Maybe this is what you referred to regarding the application dealing with tabs?

moz2u said

I'm curious why, in Options, Applications, there is

PDF	Use Foxit Reader
Portable Document Format (PDF)	Preview in Firefox

The second one corresponds to the application/pdf content type and is the built-in one. The first one must correspond to some other Content-Type.

However, your Options/Preferences page doesn't match up to the code you posted from handlers.json. Preview in Firefox is action 3:

  "application/pdf":{
    "action":3,
    "extensions":["pdf"]
  }

Are you sure that was the correct handlers.json file?

Now I notice that if I click the downward arrow on PDF (1st one) and choose "Use other" and select Firefox (since that is the fastest and seems to work just fine), it doesn't store.

Please do not tell Firefox to "Use other" and open Firefox to open a binary download like a PDF. That can cause an infinite loop: Firefox repeatedly opens empty tabs or windows after you click on a link. The ONLY safe setting for viewing a PDF in Firefox is "Preview in Firefox".

Also in the 2nd one it offers "Preview in Firefox" but that is not offered in the 1st one. Maybe this is what you referred to regarding the application dealing with tabs?

The first one is not application/pdf so Firefox will never suggest "Preview in Firefox" for whatever that is/was.