I’ve had this same experience on Linux Mint. I’ll run apt update & apt upgrade and, occasionally, if Firefox is one of the things being updated, new tabs and new pages won’t load and will tell me I need to do a system restart to continue browsing.

I always update manually, so it never happens without me initiating the update first. But sometimes I’m like, “Dangit, didn’t realize this update would require a restart to keep using Firefox.”