Wow, I experienced this yesterday while I was absentmindedly using my computer.... I assumed I had clicked something without realizing it. The idea that it was an intentional pop up didn't even enter my head.
For me it hit as a double-whammy. I tried opening a new tab, but had to stop what I was doing to restart Firefox instead because of a Snap[1] update, then I got this immediately after Firefox started back up. A really nasty snapshot of where free software is at in 2023.
I went through the song-and-dance for several versions to remove the Snap firefox and go back to the deb, and now that the deb is gone I just downloaded it directly from mozilla so it can use its own internal autoupdate feature. It works better that way anyway.
[1] https://en.wikipedia.org/wiki/Snap_(software)