Hi BostonBowTie, as you know, Firefox 52 ESR is no longer supported or receiving security updates.

Thank you for sharing your browser data. The plugins list only has Flash, not Adobe Acrobat, so that's the first thing you need to sort out. What version of Acrobat do you have? Can you make sure it is up-to-date: in your Adobe product, go to the Help menu and click Check for Updates. After updating, restart Firefox to see what it can find.

Normally Firefox will discover the Adobe PDF plugin through an entry the Adobe Acrobat/Reader installer adds to the Windows registry. Here's a copy/paste from an old thread on how to investigate:

One possible reason for an installed plugin not to appear on the Add-ons page is if Firefox has been set NOT to scan the registry for plugins. Another is that Firefox's settings file storing plugin data has become corrupted. Here are the steps to check those possibilities:

Confirm that Firefox is scanning the registry for plugins

Firefox finds the plugin from a registry entry. Some users have the scanning turned off for some reason. Here's how to check and fix that:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button accepting the risk.

(2) In the filter box, type or paste scan and pause while the list is filtered

(3) If plugin.scan.plid.all is not set to its default value of true, double-click it to toggle it back to the default.

If you changed this setting, exit and restart Firefox to trigger another scan.

If you check the Add-ons page, Plugins section again, can you find "Adobe PDF" there?

If Firefox is set to scan but still does not find Flash and list it on the Add-ons page...

Remove Firefox's plugin registry file

Sometimes the pluginreg.dat file becomes corrupted. You can remove it and have Firefox re-scan for plugins.

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

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

In the first table on the page, click the "Show Folder" button to launch a new Windows Explorer window showing your settings files.

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

• "3-bar" menu button > "power" button
• File menu > Exit

Pause while Firefox finishes its cleanup, then rename pluginreg.dat to something like pluginreg.old

Start Firefox back up again.

When you return to the Add-ons page, is "Adobe PDF" finally listed?

You wrote: I had version 50 or so set up this way, and then accidentally "updated" and had to reinstall from scratch.

To what Firefox version did you update accidentally?

If you used the profile with a Firefox 55+ version then files and folders (storage) might have become incompatible with older Firefox versions.

Thanks jascher.

I have Adobe Acrobat X Pro 10.1.16, and it is fully updated.

I set Tools > Options > Applications > PDF set to "Use Adobe Acrobat" (I've done that several times in the last couple weeks, and it keeps reverting to "PDF > Save As" -- can we make that stop too?)

When I go to

  http://cafc.uscourts.gov/opinions-orders
and I right-click a case to "Open in new tab" I get --
   -- a new tab, with the URL in the address bar, but white contents
   -- Acrobat opened as a new window

What I want is what I got before the PDF viewer was added to Firefox, I want 2007 --

   -- right click on a case, with "Open in new tab"
   -- it opens in a new tab, with the URL in the address bar, and Adobe Acrobat in the body of the window

Yes, I have 52.9.0 ESD. I know it gets no updates. That is the choice. In a world where I only use my computer to get work done, often on a deadline, and I have to choose between stability vs some security risk, feature updates are an evil to be avoided. Stability is worth more to me. So I choose to live without security updates, and reply on a paid-for antivirus.

If there were an extended extended extended support version that got security updates only, and NO stability-disrupting updates EVER, I'd do that. But I don't have that choice. So I want no updates.

Hi BostonBowTie, "Use Adobe Acrobat" with an Adobe logo generally refers to the external application. When Firefox is being set to use the plugin, there is a Firefox plugin icon and it mentions viewing "in Firefox." Here's an old screenshot from the gallery:

If you do not have that option, then Firefox is not instantiating the plugin. That could be because Acrobat X and XI are deprecated now. Or it could be one of the problems I mentioned in my earlier reply.

The Extended Support Release of Firefox gets feature updates annually. That's the most stability available. https://www.mozilla.org/firefox/organizations/