About time Rated 5 out of 5 stars
Ok so I haven't even tried this yet, but I'm giving 5 stars for the concept alone.
I like to create a new default profile when there's a new version of Firefox released (not incremental updates) and it's a major chore rebuilding my add-ons (I use a lot) so I'm pretty psyched about your extension and barring any major issues (I doubt they'll be any) I predict that Massive Extender is going to be pretty popular.
Not only for the purpose that I mentioned, but it will allow others to compile collections to install or install whole collections for friends etc.
By the way,
You can use the AMO forums or the mozillaZine ones as a place to provide support for your add-on's users. Just write a post about your add-on and add a link to that post here on AMO for your support link.
Add-on Releases (mozilla Add-ons Forum)
Extension/Theme Releases (mozillaZine Forums)