Hi, it sounds like your AV software monitors Firefox's connections and is blocking that request. Since the request is blocked, there is nothing downloaded to disk and nothing for a regular scan to find. Success!

As for why Firefox requested something from an ad server, usually this would be because a page you opened gets paid based on showing content from that server. Can you recognize any pattern as to when this block notice appears? Is it associated with a particular site?

If not, maybe one of your add-ons is trying to retrieve ads. You could test in Firefox's Troubleshoot Mode, where Firefox temporarily deactivates extensions, hardware acceleration, any userChrome.css/userContent.css files, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is running:

You can restart Firefox in Troubleshoot Mode using either:

• "3-bar" menu button > Help > Troubleshoot Mode...
• (menu bar) Help menu > Troubleshoot Mode...

and OK the restart. A small dialog should appear. Click the Open button.

If Firefox is not running:

Hold down the Shift key when starting Firefox. (On Mac, hold down the option/alt key instead of the Shift key.) A small dialog should appear. Click the Open button.

Note: Don't use the Refresh without first reviewing this article to understand what will be deleted: Refresh Firefox - reset add-ons and settings.

Any improvement?

Ref. Diagnose Firefox issues using Troubleshoot Mode

Apparently it changed the problem, but I guess that now all of the adons are disabled. Prior to this several panes would open up and as quick as I could close them another would open up. This would continue for several iterations and then quit for a while only to return and go through the same routine. After I performed the procedure that you gave it only happened once, so I guess that defines where the problem source is, but what do I do next to allow normal operations? Or do I need to delete all adons and reinstall them one by one?

As a first step, minimize your extensions just to the essentials. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• click the Extensions button (puzzle piece) on the toolbar > Manage Extensions
• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. On the right side, find the "Manage Your Extensions" heading.

If there is at least one extension before the next heading -- "Recommended Extensions" -- please continue:

Then cast a critical eye over the list below that heading. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious or that you just do not remember installing or why? If in doubt, disable (or remove). For your privacy and security, don't let mystery programs linger here.

Any improvement?