Firefox has required a browser restart after an update is installed for years. It manifests as an inability to open new tabs, with a prompt to restart. This is a safety mechanism implemented to avoid strange bugs that could happen when mixing code from the old and new versions.

As far as I know, Firefox does not have any mechanism to forbid running an old version, as your title is implying. You probably left auto updates on, and got that dialog I described?

You are right - I just didn't see that for years, and didn't bother to search for it.