Well, we actually do. E.g. NaCl requires at least Chrome 15.
But features of that scale are rare in modern browsers. That is why the perceived ability to get rid of version numbers, because "releases" don't bring a lot more than "twitter searches" and similar stuff that might have been implemented in an addon.
Browser versions will reappear, when
a) major progress restarts in that area and third parties will need to target specific versions, not being able to catch up with pace of development
b) "Firefox"-no-name brand gets old and someone will come up with "Firefox II" (or FireAsaDotzler, or whatever) to "refresh" the franchise.