It's the same as Chrome's - 6 weeks. What I think made a lot of people before attack Mozilla for it was because: 1) the "Firefox user" wasn't used to this kind of fast updates, the way the "Chrome user" was - and 2) Firefox' updates were not silent, and therefore you knew you got an update and some users may have been getting annoyed having to update it so often, instead of silently upgrading in the background. But apparently they now have silent upgrades starting with this version.
This is one of the biggest problems to tackle when updating software: How can the user be uninterrupted? This is one of the biggest problems I have on my WP7. My apps stay out of date for a long time until I manually go update them. I don't think to manually update them very often (maybe once a month). I'd much rather the app be updated silently in the background, and when I re-open it after the updates have been downloaded/applied, I then notice that things have changed.
This is pretty much the same way websites operate and you don't have much control over it. You may be able to prolong the update while they are "beta" testing it (a la Google/Facebook), but eventually you get the update and have to deal with it.
I like silent updates when they're not likely to be huge user-visible improvements, mostly performance enhancements and bug fixes. In the case of Firefox or Chrome, for example, I'm perfectly happy with the current feature set and just want them to keep getting faster and more stable. On the other hand, most of the Android apps I use are missing a couple of functions or have some annoying visisble bug I am waiting to be fixed. Then I prefer updates to be automatic but not silent so I can read the change list and if something I am waiting for has been added or fixed I know (and rejoice) right away.
