Rated 4 out of 5 stars
Decent add-on, I think it's the only one that lets you do this sort of thing - editing cookies.
At first I thought this add-on didn't work because I tried to change the "Path" attribute and the changes wouldn't save. (If that's what you're trying to do, write down the current cookie values, delete the cookie, then add it with the new Path value you want.)
However, I later discovered that I was able to edit the Content and expiration date fields just fine, so I guess this does work more-or-less after all.
One other thing: After installation, you can find this add-on in Tools -> Cookie Editor. I guess that's obvious in retrospect, but the first place I looked was for an extra button or menu option in the standard cookie window - nope.