This is a tempest in a teapot. When you upgrade to a new version (or a patched version) you get notified of new things. This is one of the new things they want to notify people about, because the browser is free and they need a way to make money or they will perish.
This is not shown as content inside the tab that gets opened on launch after an update. I wouldn’t have a problem with that, and I suspect most other people wouldn’t either. I wouldn’t have much of a problem if they launched that tab when the browser wasn’t updated, but they had a new thing to promote (as long as it’s sufficiently infrequent). This is not that.
This was shown over the top of the active tab containing a website unrelated to Mozilla products, and it disabled the tab bar and the menu. The exact conditions to trigger it are not obvious to me, but it appears to involve idle time.
I’m not seeing it on my Mac. Updated to the latest version 2 days ago, no pop-ups so far.
From what I’ve been able to gather, it was only in use for a brief period, and only appeared after Firefox was left idle for 20 minutes.
Interesting, I still haven’t seen it, but Mozilla have pulled the ad.