On Linux:

• an application’s files can be updated while the application is running, and
• there’s an OS-wide updater (i.e. package manager) with which you can update most software, including Firefox. (You can also get Firefox with its built-in auto-updater, but most people prefer the OS-wide updater.)

Both of these are good things. But Firefox, with its relatively advanced multi-process architecture, had a problem here, because it could happen that its files got updated while it was running and then when it started a new process, this new process might be incompatible with the old processes, therefore unable to communicate correctly.

Their initial solution was to force you to quit Firefox and reopen it, when they detected that the files had changed and you did something in Firefox which might need a new process, so primarily when opening a new tab.

I’m guessing, they now implemented a way to launch the new process by still using the old files from before the update.