No, Linux. I don't know what it's doing in there. Lots of disk I/O. Clearing the "startup cache" can help.

My guess: something is seriously borked in your profile. Easy to test.

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.

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?

Maybe try creating a new profile? I've had cases where a profile can cause Weird Shit™ to happen. Kind of annoying though. Probably something in a SQLite database or some such, but I didn't have the interest to track it down.

I've experienced this since 2018 across all versions of linux mint since 2018.

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.

I'm noticing it on a fast SSD, though it's more like 5-10s after launching. No issues once it's running. I'd guess it's related to my very old and large profile.

Not sure if it will help but about:processes might give some more info about what is causing the activity.

Dude, I have literally 4,000 tabs (not a joke), and my Firefox is fully loaded on boot after only a couple of seconds.

Something is wrong with your system.