One of the statements that keeps getting thrown around in the discussion about the Firefox rapid release process is that because the releases are every six weeks, there isn't really enough time for any major changes. This statement is patently false, and there's an easy way to prove that - the Firefox button.
The Firefox button was a huge change for Firefox 4. I would wager it cost (or will cost) companies across the world tens of thousands of dollars (or more) in rewriting of support scripts, redoing of screen shots and actual support calls.
Let's assume for just a minute that the switch to the Firefox button had been done within the context of the rapid release cycle. How would it have happened? Could it have been phased in? Could it somehow have been done across multiple releases? No. The answer is that between two "minor" releases, the change would have simply been made. What's the takeaway?
Whether or not a change is major or minor has nothing to do with time. It only has to do with the change.
So how do you mitigate a situation like this? I'm glad you ask. You do what Microsoft did and provide a Group Policy (or preference) to turn on the menu bar by default. This allows the browser to be deployed without taking this change. I received quite a few emails when Firefox 4 was released with folks asking this specific question. I had to tell them that there was simply no easy way to do it in Firefox without writing an extension. I considered adding it into the CCK Wizard, but simply haven't had time.
We'll finish with a story. Last week my wife finally bit the bullet and upgraded from Firefox 3.6 to Firefox 4/5. She's technical and has been using web browsers for 10+ years. Her first comment? "Where did my menu go?" I said, "Everything is in the Firefox button now." She said, "I want my menu back." So I showed her how to get rid of the Firefox button and go back to the old menu.
You're messing with 10+ years of muscle memory here. Change is hard. And expensive.