I used to hop between browsers often, and for a few years Firefox felt significantly slower. Does that really matter that much? I don't think so.
I often think that Firefox never really got bad, the difference between FF and Chrome was never massive. It's a bit unfair that FF lost so much market share for that small difference.
If one browser is 2% faster than the other, and the cost to choose one or the other is near zero, then expect huge switching.
... And that's before we factor in the far more dominant network effects, like "What comes pre-installed" and "which integrates with my accounts and syncs my aggregated data and preferences history onto a new machine transparently."
If nobody else is using that memory, they might as well.
:'[
The serious answer is hopefully nothing.
1/ yes its depreciated on paper, but it still works