I have run Firefox on Linux for decades (and a few extensions, and metric gobs of tabs), with zero cases of the behaviour you describe.
Maybe their distro has a broken Firefox package, they messed with the default installation, have too many extensions, or malware? A slow mechanical disk?
The problem appeared to be a lot of unnecessary disk io coupled with DNA lookup that only get done after every single read request is complete. This means that when tab #10 is taking long to read whatever from disk it blocks every other tab.
Noticeable only when using spinning rust disks.
Something is wrong with your system.
