Mobile is different. I rarely even use mobile. I do however use a lot of tabs on desktop, and Firefox is found very wanting in the performance department.

I don't even think its about number of tabs. Just yesterday and today, Mullvad browser takes minutes to load a set of about 7 pinned tabs (with no other tabs) on startup, whereas Helium (which is based on Chromium) loads in a second or two close to a hundred tabs.