It's a pretty ridiculous scheme done just to copy Chrome, but it also doesn't do much harm in practice. But browser version numbers no longer really have any useful or memorable meaning.
I have no fucking[0] idea if Firefox 99 is just a minor upgrade over Firefox 98 or it would completely change the UI and my UX with it. With X.YY to X+1.00 I could, at least, be warned what something crucial could change. Nowadays I'm the hostage of "We think you need to do things THAT way" of whatever fever at Mozilla corp at the moment.[1]
[0] I mean it.
[1] There was something about themes or colours or whatever in the last ten updates. Did you used it more for than a day?
They each release on a regular schedule. I don’t think that they have the same schedule. You can see each of them incrementing with each release. They have been doing this for several years now. It really is just chance they they are at 100 in roughly the same time.