Thanks to chrnno Rated 4 out of 5 stars

@chrnno: thank you very much for your review. Your suggestion worked perfectly.
Anyway, there's no need to uninstall Brief; simply close Firefox, delete all brief* sql files from the profile (or rename them) and restart Firefox.

This review is for a previous version of the add-on (1.6.2). 

Does not work with FF17 Rated 3 out of 5 stars

Project seems dead: support forum doesn't work and latest versions (stable 1.6.2 and beta 1.7a1) don't work with FF17. All feeds are empty, even if the unread count near each feed's name is not zero.

This review is for a previous version of the add-on (1.6.2).