Rated 3 out of 5 stars

Tried with 3.1 beta3 and seemed ok.
I would like to see
1- Ability to see or export "cleaned" css code as in html validator.
2- An option for auto scanning so I wouldn't need to hit scan at every page.

This review is for a previous version of the add-on (0.0.5). 

export: no, autoscan: yes

1. This might be a bad idea since it is very likely you'll throw away some CSS that is only used in extreme situations (for error messages or whatever)
2. Agreed it would be nice, it's on the todo list.