Sometimes plugins seem to preempt external viewers despite your preference. This could indicate a corrupted preferences file, but as a shortcut to troubleshooting that, if you never want to use the Adobe Plugin, you can set it to "Never Activate" on the Add-ons page. Either:

• Command+Shift+a
• Tools menu > Add-ons

In the left column, click Plugins. It should be near the top of the list. Does that work?

No.

My apologies, as I had failed to mention that I had already set it to never activate, and checked that.

I should mention that the OS correctly uses Preview when I open a PDF. Also, I have tried restarting Firefox, too.

So, how do I un-corrupt the preferences file?

The file that stores your preferences for how to open downloads is named mimeTypes.rdf. Since it has a somewhat inscrutable format, the usual remedy is to rename it and have Firefox generate a new one.

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

Help > Troubleshooting Information > "Show in Finder" button

Leaving Finder open, switch back to Firefox and Quit

Pause while Firefox finishes its cleanup, then rename mimeTypes.rdf to something like mimeTypes.old

Restart Firefox and reselect your preference. Any improvement?

I cannot locate any file named mimeTypes.rdf.

Perhaps it has a different name?

Did Firefox open the correct folder? You would find very up-to-date files such as:

• places.sqlite (database that stores bookmarks and history)
• prefs.js (file that stores most of your Firefox preferences)

I only have experience with Windows, but I'm not aware of any difference in file naming conventions on Mac.

I simply tried to use the OS to find a file with that name anywhere under my home directory and all its sub-directories. When that failed, I tried the search on the whole computer, with same null result.

I do find file names containing the following strings: mim place prefs. So, the search utility seems to be working. However, if I get more specific, then it returns a null search result.

The reason you can't find the file in a general search is that your application folders are hidden by the OS. Please use the "Show in Finder" button on the Troubleshooting Information page as I described.

Ah yes. I was following your instructions too literally and not recognizing that I was looking for a hidden file. It works fine now. Thanks.

By the way, the button that says "mark it as solved" takes me to a Mozilla "page not found" page.