Hello, CHellner!

I know a lot about this topic. I answered a similar question few days ago. here is what I said:

I read an article on the Mozilla Support Page and it says that Firefox Quantum will no longer be supporting NPAPI plugins like Adobe Flash, Shockwave Flash, and Silverlight, because there are more and more static pages on the internet, and I think that a solution to this problem is to downgrade to Firefox ESR or older. (but don't got back to those really old versions like 10.0) The page with the different versions of Firefox is https://ftp.mozilla.org/pub/firefox/releases/ . I recommend scrolling down to Firefox 52.0 and installing that.

You also may need to load the plugins manually, so you can access that page at https://support.mozilla.org/en-US/kb/npapi-plugins

I hope it works!

ThePriusMan said

Hello, CHellner! I know a lot about this topic. I answered a similar question few days ago. here is what I said: I read an article on the Mozilla Support Page and it says that Firefox Quantum will no longer be supporting NPAPI plugins like Adobe Flash, Shockwave Flash, and Silverlight, because there are more and more static pages on the internet, and I think that a solution to this problem is to downgrade to Firefox ESR or older. (but don't got back to those really old versions like 10.0) The page with the different versions of Firefox is https://ftp.mozilla.org/pub/firefox/releases/ . I recommend scrolling down to Firefox 52.0 and installing that. You also may need to load the plugins manually, so you can access that page at https://support.mozilla.org/en-US/kb/npapi-plugins I hope it works!

That also involves PDF viewing (Silverlight)

NPAPI support in Firefox ended long before Quantum Firefox 57 arrived on the scene.

Firefox 51 Release and the 52 ESR version were the last to support NPAPI.

The Firefox 52 ESR version goes EOL (End Of Life) on Sept 5th, so that version isn't a "solution" for more than a few more months (unless Mozilla extends that deadline with a 10th security update version). https://wiki.mozilla.org/Release_Management/Calendar

That explains the problem. An automatic update took place yesterday and that is when it stopped working. Now I have to decide whether I need to convert to chrome or go through the extra step of downloading the PDF and then telling it to open. DO NOT understand why a workaround cannot be done in Firefox when chrome and explorer do not seem to have a problem with it.

Hi CHellner, was it previously opening in Foxit Reader outside of Firefox, or in a Foxit plugin in a Firefox tab? As noted in earlier replies, no plugins other than Flash will run in a Firefox tab anymore. However, you still should be able to open a PDF in your choice of external programs.

If that is the issue:

If Firefox stops obeying the Application settings listed on the Options page, you may need to remove a settings file and have Firefox regenerate it. Here's how:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > Exit/Quit
• (menu bar) File > Exit (or Firefox > Quit)

Pause while Firefox finishes its cleanup, then:

• rename handlers.json to something like handlersOLD.json
• if you see a file named mimeTypes.rdf, rename that one to mimeTypesOLD.rdf

Start Firefox back up again and it should be back to the default setting:

Portable Document Format (PDF) => Preview in Firefox

You can edit that to your preferred behavior on the Options page.

Any improvement?

Note that Firefox will only use this setting if the server indicates that the content-type is application/pdf. If the server says it is something else, then you would normally get the Open / Save / Cancel dialog.

the-edmeister said

NPAPI support in Firefox ended long before Quantum Firefox 57 arrived on the scene. Firefox 51 Release and the 52 ESR version were the last to support NPAPI. The Firefox 52 ESR version goes EOL (End Of Life) on Sept 5th, so that version isn't a "solution" for more than a few more months (unless Mozilla extends that deadline with a 10th security update version). https://wiki.mozilla.org/Release_Management/Calendar

The support for Firefox 52.0 shouldn't really matter so you shouldn't worry about the End of Life.

(I am running Windows Vista which doesn't support Firefox 53+)

This is exactly what was needed. Thank you very much. I am back up and running. Thank you again for preventing the switch to crome.

jscher2000 said

Hi CHellner, was it previously opening in Foxit Reader outside of Firefox, or in a Foxit plugin in a Firefox tab? As noted in earlier replies, no plugins other than Flash will run in a Firefox tab anymore. However, you still should be able to open a PDF in your choice of external programs. If that is the issue: If Firefox stops obeying the Application settings listed on the Options page, you may need to remove a settings file and have Firefox regenerate it. Here's how: Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in Windows Explorer. Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > Exit/Quit
• (menu bar) File > Exit (or Firefox > Quit)

Pause while Firefox finishes its cleanup, then:

• rename handlers.json to something like handlersOLD.json
• if you see a file named mimeTypes.rdf, rename that one to mimeTypesOLD.rdf

Start Firefox back up again and it should be back to the default setting: Portable Document Format (PDF) => Preview in Firefox You can edit that to your preferred behavior on the Options page. Any improvement?

---

Note that Firefox will only use this setting if the server indicates that the content-type is application/pdf. If the server says it is something else, then you would normally get the Open / Save / Cancel dialog.

ThePriusMan said

I recommend scrolling down to Firefox 52.0 and installing that.

So you were recommending a fix that wouldn't stand a chance of helping this user. Nonsensical.

ThePriusMan said

The support for Firefox 52.0 shouldn't really matter so you shouldn't worry about the End of Life. (I am running Windows Vista which doesn't support Firefox 53+)

It does matter - Firefox 52 hasn't been supported since Firefox 53 was released. Whereas, the Firefox 52 ESR version is still supported by Mozilla and will get its last Security Update on June 26th with Firefox 52.9! Unless Mozilla changes its mind over the next few months, which I doubt will happen because the next ESR version was already postponed 6 weeks from Firefox 59 to Firefox 60.

And just because you use an EOL Operating System (Vista) and recommended a no longer supported browser version (52) doesn't mean contributors should be recommending that to other users; especially those users who are using Windows 10 and the latest version of Firefox.