I think that what Chrome (and later Firefox) ended up doing is a pretty good compromise. The major version of Chrome is basically "Chrome" - if they ever break compatibility in any kind of major way, it probably will be as a different project. Firefox is the same way.
I know that it'd be great if you could derive deep secrets about the changes in each release from just the version number, but that just doesn't scale to complex projects. Use SemVer and similar schemes where they're appropriate, but don't treat them like they're the only right choice.
At the end of the day when the version number isn't really known to the end user it makes little difference if you bump up the major version number or the minor.
Also I love how quickly we get new features now. Firefox has improved more in the past year than it did in the past several years before the shift to a 6 week release cycle. It isn't suited to all software I admit but for a browser it is perfect. None of this "This site only works with Firefox XX or Chrome XX". Thank god!
This is more caused by Firefox stopping to break the compatibility all the time. The plugin API in Firefox used to be just the internals, exposed to JS, so everytime something changed, it broke extensions. These days, they stopped doing that quite as often plus they published the Jetpack SDK which promises a stable API at the cost of less possibilities to change the browsers behaviour.
Where is the meaning? At least for Chrome/Firefox you know x+1 means another six weeks are over...