This happens if another instance of Firefox or an external package manager updates Firefox while you are running Firefox. This can happen if you have installed

This happens if another instance of Firefox or an external package manager updates Firefox while you are running Firefox. This can happen if you have installed Firefox via a Linux package manager, or if you are running Firefox with multiple profiles. While I understand that it is very annoying if you hit this issue, it is not done as some kind of trick to try to get you to update. The issue is that Firefox is trying to start a new content process to load your page, but the old executable no longer exists so it is impossible.