There are probably tens of millions, possibly hundreds of millions of people who downloaded some version of Firefox, tried it out, and went back to their default browser. If the length of time since their last attempt at using Firefox is greater than some number of days, we should offer to let them "start fresh" where we re-migrate from their default browser into a clean profile directory. It would be exceptionally cool if we could also somehow replace their actual Firefox install with a brand new one to ensure we've cleaned out any cruft in the install directory that may have interfered with their earlier trial.

Sorry, I left off the two justifications I had in mind when suggesting this. One, if it's been a long time, say you tried Firefox 2.0 and went right back to IE and now you're willing to try out 3.0, you've ammassed a crapload of new profile data in IE and Firefox will use the original ancient migrated information which can't possibly be up to date. Two, if you gave up on Firefox because of some performance or reliability problem and you are willing to give the new version a try, it'd be nice to make sure we don't re-break the new Firefox with cruft in the profile or install directory. If we pick the right number of days before offering this, I think we can be assured that people will mostly appreciate it. I don't know what that number is, but I'd put it at as little as 2 or 3 months and no more than 6 or 7 months.

This could be added as another option to the existing UI from bug 498181.