Yeah, kind of unfortunate that it didn't do much at the start and people were asking themselves why it's there, but from a development perspective, it absolutely makes sense that they wouldn't implement everything ahead of time, before getting feedback from users...
Well, they're rolling out their Shopping sidebar and ad network really quietly. Been rolling it out since 5 versions ago. But something tells me they don't really want feedback on that, so who knows.
For others who were confused, it's the icon to the far left of the tab bar.
I've used it on my Android phone (open list of tabs, it's at the top next to the private tab selector), but never realized it was also available on desktop Firefox.
I actually hid it after thinking it wasn't very good, but that was several versions ago (with a different icon). Thanks for pointing that out, and I updated my comment!
In the rapid release model there are no updates bigger than the regularly scheduled releases. So each regularly scheduled release needs to bump the biggest version number. Otherwise the biggest number would never change and there would also be fewer ways to distinguish smaller releases.
That's not necessarily true, they could totally use semantic versioning (but without the requirement for a breaking change) and bump the major version when there's a big user-facing change. For example, Firefox Quantum would've been a great time for a version bump. Or they could do with Year.Release versioning like Ubuntu does.
I think either of those are more useful than the current sequential numbers because they provide some additional information. I like the Major.Minor style so I have an idea of how significant the changes are.