"how do I seeing the same" should be "how do I avoid seeing the same"

Usually this is caused by one of your tabs initiating the download when it is restored. Bookmark your tabs, then close them and see if it still happens.

Thanks but that didn't do anything. I don't open Firefox with tabs from the last session. I just this moment opened it to https://Wikipedia.org and to my own http://localhost:8080 and the same thing happens. An irritating mystery. Surely Firefox has some sort of memory about this particular file hidden somewhere in its bowels but I've not been able to find it. Nothing showing in ProgramData belonging to Mozilla.

Hey there! It sounds like you're having a bit of trouble with those persistent virus/malware warnings in Firefox. I get how that can be annoying. Let's try a couple of things to fix this. First, let's clear your browsing history and cached data. Go to the menu in the top-right corner, select "Options," then "Privacy & Security," and hit "Clear Data." Next, make sure your Firefox is up to date. If it's not, updating might help resolve the issue. Also, check if any extensions or add-ons are causing the problem by disabling them one by one. Lastly, try creating a new Firefox profile to see if the issue persists. You can do this by opening the Run dialog (Windows + R) and typing in "firefox.exe -P" without the quotes. This will open the Profile Manager, where you can create a new profile. If none of these work, please don't hesitate to reach out for more help.

I nuked all data and files I could locate -- wiped everything which could be really painful but no help. Disabled all extensions but no help. I am running the latest 17.0b3 (64-bit) but I've been running betas with nary a problem for years and years now. I'm kind of worried about going the new profile route though -- what will I lose and what kind of work will it take to get back to a functioning browser?

It's totally understandable to be worried about starting with a new profile, but sometimes it can really help fix stubborn issues. Going that route might mean you lose some personalized settings, like bookmarks and extensions. But don't worry, you can always export and save those things first. Once you've got a fresh profile, you can start adding back your extensions and settings gradually, checking if the issue returns. It might take a bit of time, but it's a good way to isolate what might be causing the trouble.

You can try to open a new window and possibly move important tabs to this new window and close the window that causes this message to show.

An easy way to test a new profile is to install Developer Edition and see if it happens there. If that works then you might need to refresh your existing profile.

I think, based on tenuous evidence, that the problem is corrected although I cannot fathom the reason. Just now I downloaded an image from the New York Times and it worked. Now, when I open a fresh instance of FF the warning no longer appears. Did the new bit of download history somehow nullify the old bogus warning? It looks that way and if the "fix" holds I'll come back and mark this as solved. Thanks to everyone who offered suggestions.