It is possible that (anti-virus) security software is causing the problem.
Try to disable security software temporarily to see if that makes a difference.

Already tried that. No dice.

Does it matter where you specify to save the file, such as the default Downloads folder versus a folder outside your user profile (e.g., C:\whatever) ?

If you select Open in the Download dialog instead of Save, Firefox should drop the file into the system temp folder. Does it stay there any longer than another folder?

It does not where the downloads go. And no it does not.

Could you try this:

New Profile Test

This takes about 3 minutes, plus the time to test some downloads.

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Click the "Create a New Profile" button, then click Next. Assign a name like June2020, ignore the option to relocate the profile folder, and click the Finish button.

After creating the profile, scroll down to it and click the Launch profile in new browser button.

Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system so we can get a clean test.

Do downloads save any better in the new profile?

When you are done with the experiment, you can close the extra window without affecting your regular Firefox profile. (June2020 will remain available for future testing.)

IMPORTANT: Creating a new profile may change your default. If your regular profile has a Set as Default Profile button after this test, click that to make sure your next startup doesn't yield a surprise.