Firefox and Chrome already release on a fixed date schedule instead of a feature based schedule. The proposal is to simply use the date in the version directly instead of indirectly.
"Major release" also doesn't matter like it used to. One of the big new features in recent versions of Firefox is WebRender, but that is being slowly rolled out among users based on CPU/display/platform, it's not a Big Bang feature release.
There is no right way to decide what is a major release. Whatever you do, some users will think a major release should have been classed as minor, and vice versa. It's ultimately easier for everyone if you don't make the distinction.
