Earlier quoted context omitted.
> and presumably on other OSs Nope, it only happens when something outside changed Firefox's files (in this case when Ubuntu swapped files because dpkg updated Firefox). This never happens* in Windows and macOS (it might nag, but you can definitely dismiss it). It seems that johnchristopher has a suggestion to disable auto-updates on a specific application in Debian and Ubuntu (haven't tested it though): https://news…
This is exactly right. And this happens because it needs to load libraries or exec subprocesses that aren't compatible between versions. Since the matching version of the file has been replaced with a newer one it doesn't really have a choice, it is unable to launch the new tab. The nice message is a better alternative than crashing. This also doesn't occur on NixOS because the new version is in a different direcotry…
Also with flatpak; you are using the old version until you close it. Then it will be garbage collected. On the next launch, you will be running a new version from different root.