Your computer must be restarted for the change to take effect.
We used to joke about Windows 95 and it’s ridiculous reboot requirements. The line we used was:
“You have moved an icon on your desktop. Windows must be restarted for the change to take effect.”
Those were not the days, and I thought that they were pretty much over.
I can’t think of any circumstances where a reboot should be necessary to complete the installation of application software. That was last century. I’m OK with reboots for things like kernel updates, firmware updates, and perhaps even driver updates.
But a browser?
It’s possible that the reboot is being forced by non-Mozilla browser add ons – I don’t have any way of knowing. But if an add on to an application can force the application to force the operating system to reboot, then the application and OS design are both defective.