Then it sounds like Firefox has the "restart page" to mitigate this exact edge case. Better than having undefined behavior like that imo.
As I get it from other comments, this is not a problem with the built-in updater (as on Windows). On linux, when updating via package manager, you should now this can be an issue with any program. Yes, most programs survive running while being updated, but for a complicated piece of software (like a browser) this behavior is understandable.
On linux I use kill -9 before any update, on the parent of ff's entire process tree. Works for me every single time, reliably getting my tabs back without the need for any extension.
As I get it from other comments, this is not a problem with the built-in updater (as on Windows). On linux, when updating via package manager, you should now this can be an issue with any program. Yes, most programs survive running while being updated, but for a complicated piece of software (like a browser) this behavior is understandable.
See the Arch wiki for updating on Arch: https://wiki.archlinux.org/title/System_maintenance#Restart_...