Same here, vanilla Firefox snap on Ubuntu. If anything, Firefox with hundreds (literally) of tabs starts way faster than Chrome with 10, thanks to lazy its loading. RAM usage has always been stellar in Firefox, in my experience.
Maybe their distro has a broken Firefox package, they messed with the default installation, have too many extensions, or malware? A slow mechanical disk?
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.