Great add-on! Two suggestions. Rated 4 out of 5 stars
This is a great tool! It's nice you can copy the selection, instead of only saving it, like another add-on does.
I do miss one thing (using FF 9.0.1) and that's the option to adjust the selection. If there were some handles around the selection to adjust its dimensions that would be great!
And like Lilyca mentioned, and indication of the dimensions would be a super addition as well.