Get the OneTab extension. It'll save and close all those tabs. That way you won't have Firefox crashing during startup once you exceed the number of tabs it can handle (a few thousand).

Tip: the crashing is caused by certain extensions such as OneTab and All Tabs Helper which for some reason seem to cause all the tabs to load, just when restoring a session. Temporarily disable these extensions before restoring, then you can reenable.

I have 117 thousand tabs, and it starts up fine. Just adjust your shm ratio.

(I'm kidding)

I've had it function just fine around 9000 tabs.

Doesn't Firefox natively unload tabs these days?

You can also just do tab groups in ffx

Amateur numbers... I've tested over 10000 (not right now)... It used to get really slow after 9000, but things seem to have improved.

Were all tabs loaded though? If one tab takes 10 Mb of RAM (very low estimate; many take 50-100 Mb, especially Youtube), then 10K tabs require 100Gb.

I’ve started seeing tabs that weigh in at multiple gbs. Cloud provider consoles are particularly egregious examples here.

I use Chrome and have 1500 tabs on my MacBook Pro. I'm a packrat.