I've been writing a blog post about this in my head for a while, but after glazman's post, I definitely feel I need to weigh in.
My opinion of the new rapid release process depends on which hat I wear. So I'll offer three opinions.
As a Firefox developer, I love it. One of the things that frustrated me the most about Mozilla process in the pass is that security release only contained MAJOR fixes. So even if there was an important fix, it had to wait for the next release train which could be a year. This contributed to people complaining constantly about bugs even though they were fixed. So faster releases means faster fixes and less bugs for people to report.
As an add-on developer, I find it annoying but mostly a non-issue. The reason is simple: I target my add-ons for Firefox 3.6 and do basic testing on other Firefox versions. I know that seems strange, but Firefox 3.6 has 35% of the Firefox market share. So it would be silly for me to ignore millions of people. All these great new features for add-on developers are interesting, but it's unlikely I would ever use any of them until Firefox 3.6 had a < 10% share (which I don't think will happen any time soon, despite Mozilla dropping support). And the multiple versions of Firefox will make this problem even worse. I'm sure over time you'll see just about every Firefox version in use and you'll want to target the lowest common denominator.
As person involved in the corporate deployment of Firefox, I think it's a really bad idea. Companies simply can't turn around major browser updates in six weeks (and each one of these is a major update). With security releases, there was a reasonable expectation that web applications wouldn't break as a result of changes. With these releases, there is no such expectation. So a full test cycle needs to be run with every release. By the time this cycle is completed and the browser is piloted and deployed, another version of Firefox would already be released so they'd already be behind. And in the mean time, all of their browsers will be insecure, because all security updates are rolled into the major versions.
For corporate deployments, there has to be a stable branch. I bet someone is probably going to make a nice business out of creating and maintaining a stable branch...