Short version:
1.0.x will ONLY ever include high-importance fixes like security fixes, and crash fixes. In the x.y.z versioning, "z" changes are only going to contain security updates or crash fixes. "y" changes can include major changes, and no "x" change has happened yet (I don't count 0.9 -> 1.0).
Long version:
You don't seem to understand how product development works. Basically, the product has different "branches" - different copies of the code. People do active day-to-day development on the "trunk", and branches periodically split from the trunk. Take a look at
this picture (note that it's horribly inaccurate). The basic idea is that all bugs (bugs, feature requests, etc) get fixed on trunk (the big line), but when they want to make a release, they need a stable snapshot of the code. Trunk often breaks - if you follow the nightly releases from trunk, sometimes you'll get a browser with no scrollbars, one that crashes when you hit the back button, etc. It's no good for doing releases from, and keeping it clean would severely hurt the rate of development.
At some point, when it's getting near the time to do a release, they "freeze" the tree for a while - which means that all changes require approval from the "drivers" (they control where the project is going). Only bugfixes, low-risk features or other high-importance patches can be checked in during this time. After a while, a copy of the code is made (a "branch"), and trunk is reopened for regular development. The branch continues to require approval for each change - the purpose of the branch is to provide a stable platform and be a place where new bugs are not introduced (on trunk, people add bugs with features, while on branches people are not supposed to not add features). Once the branch has become sufficiently stable, a release is made. In the future, that branch will only receive high-importance fixes (major crash bugs, security fixes, etc). The idea is to make sure no bugs are introduced in patch releases (i.e. Firefox 1.0.3).
In the case of Firefox, 1.0.x was released from the Firefox 1.0 branch, which is based on Gecko 1.7. Since Gecko 1.7 was frozen, a HUGE number of changes have been made; curently Gecko is at 1.8 beta2. Eventually, Gecko 1.8 will be stable, and Firefox 1.1 will branch. With Firefox 1.1, you'll see all the changes that are made up through ~June... big changes that happen after that will not likely make it into 1.1.
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