The review team has decided to keep it in the sandbox because of a certain AMO coding standards requirement.
I'm quoting the entire reply that I got from AMO review team.
Your nominated add-on, CollapseComments, has been reviewed by a Mozilla Add-ons editor who decided to retain your add-on in the sandbox.
Reviewer: Sid Kalra
If you have questions about this review, please e-mail firstname.lastname@example.org or join #addons on irc.mozilla.org.
Thanks everyone, for taking the trouble creating AMO accounts and trying out CollapseComments 0.0.1.
Hopefully, I'll be able to complete 0.0.2 with all those features in the whishlist, plus those requirements indicated by the review team, soon.