Thanks for the report! Scan your computer for malware, Troubleshoot Firefox issues caused by malware and I've reported this sites.

Once I left the site where this occured and went back, it did not pop up again. I have scanned my system and found no malware. Returning to the site again, I did not get this popup.

Attached is a screenshot of the upgrade box I talked about in the original post.

Thanks.

http://www.airliners.net/ sets a "slider cookie" on the first load of the homepage, so as to not show the "slider" in the lower left corner of the page more than once per day (if I read the contents of that cookie correctly). That "ad" for the bogus update message may have been slipped into the "advertisement" rotation. That might be why it didn't appear again for you upon a revisit to that page.

Also, at the extreme bottom of that page, take a look at Ad Choices which has Interest Based Advertising Policies giving you a method to opt-out of Targeted Advertising for that domain, and possibly others handled by Demand Media.

Or just use Adblock Plus to block advertisements for all websites.

I frequently visit airliners.net and do not remember ever having an ad pop up in the lower left corner like this Firefox upgrade. Most of the ads are tolerable to me on this site. It was just the fact that this ad was in a different spot, when I tried to close the ad, it told me I was on release 23.0.1 which was not current and would not let me close or navigate away. That is when I collected as much info on the popup so I could post it here. Thanks for your feedback "the-edmeister"!

Is that the position (Realtime labeled ad) on the webpage that the false update appeared?

I get the exact same Realtime slide-out ad when I first visit that airliners.net main page with a different Profile that has no adblocker, and after clearing all cookies, closing Firefox, then re-opening Firefox and loading that page from history. The ad starts to "slide-out" after clicking on link on the main page, and as the new window for an advertisement is opening.

Happens for me with Firefox 20, 21, 22, in addition to Firefox 23, each with their own Profile.