I always assumed this was related to the multiprocess architecture. If you've updated Firefox in the background (e.g. via your package manager), and the new ver

I always assumed this was related to the multiprocess architecture. If you've updated Firefox in the background (e.g. via your package manager), and the new version is not API-compatible, it will not be able to create a content process for the new tab, hence the message. Obviously this is only a problem on Linux, because on other platforms Firefox's own autoupdater will only apply the update when Firefox is restarting.