This change was made because opening a new tab or loading a new page can start a new renderer process. The way things work right now is that this is started by running the on-disk renderer process executable.
If you've updated your installation, the new renderer will be the new version, but the parent (ui) process is still the old version. The protocol the two use to talk to each other is not fixed, and messages will start looking malformed. Malformed messages are handled by immediately terminating the process, because otherwise security guarantees go out the window.
So the upshot was that you'd get various crashes in the "I updated Firefox and then kept using it" situation. I've run into a bunch personally, and there is _lots_ of crash reporting data on this. The crashes are not OS-specific; they just have to do with whether changes happened to the IPC protocol.
A "proper" solution would be for the parent process to not use the on-disk version of the renderer to launch new renderers. There's work ongoing to make that possible, but in the meantime the crash volume due to this problem was high enough that the mitigation you see was put in place.
If you've updated your installation, the new renderer will be the new version, but the parent (ui) process is still the old version. The protocol the two use to talk to each other is not fixed, and messages will start looking malformed. Malformed messages are handled by immediately terminating the process, because otherwise security guarantees go out the window.
So the upshot was that you'd get various crashes in the "I updated Firefox and then kept using it" situation. I've run into a bunch personally, and there is _lots_ of crash reporting data on this. The crashes are not OS-specific; they just have to do with whether changes happened to the IPC protocol.
A "proper" solution would be for the parent process to not use the on-disk version of the renderer to launch new renderers. There's work ongoing to make that possible, but in the meantime the crash volume due to this problem was high enough that the mitigation you see was put in place.