I can see your local web servers

Other approach is to create a useful extension like:

  https://addons.mozilla.org/en-US/firefox/addon/yt-adblock/reviews/

disguise that you're inserting an iframe linking to your web server into every single page user opens, by naming variables and your tracking domain incorrectly and by waiting for an hour after installation (this may also help avoid automatic tests mozilla is doing) and then just sit back and wait and log all the referers and ip addresses. It's a bit stealthier too, but needs users to visit their local web servers. But you'll also get the full URL.

Nobody will report you or care about the report and users are banned from fixing the extension code locally even if they're able to review it themselves. Bad reviews with some actual text fade away quickly, so if someone warns your other users, it will be pushed out to page 2 after a while by other useful one word or just empty reviews and it will work out.

Is there any way to get the source code of extensions from the Mozilla web site? I think some years ago you could look at the code in your browser from a link in the version history but I don't see any links at all now.