Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

One user had this problem. Turns out the system option was set to not show extensions.

It does the same thing in safe-mode. Here is a link to a download for example (PDF). Does it download properly for you?

Example

It looks fine to me. Did you check out the bottom of my last post?

nbaer89 said

It does the same thing in safe-mode. Here is a link to a download for example (PDF). Does it download properly for you? Example

If I click that, the PDF displays in Firefox's built-in viewer. If I right-click > Save Link As, I do see a .pdf file extension.

In case it makes a difference, my Windows is set to show ALL file extensions and not just unusual ones. I recommend that change if you don't normally see all extensions. This article has the steps:

https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/

I have the hide file extension option turned off in Windows.

I have also turned off the Firefox PDF viewer and have PDF files set to save by default. If I right click -> save as the link it will keep the file extension.

I have Acrobat Pro 11 but here is the name it saves with:

nPODM_pdf

The extension doesn't show up because I have extension hidden by Windows options.