Just what I'm looking for! :D
Site URL: http://firegpg.tuxfamily.org/
FireGPG is a Firefox extension which brings an interface to crypt, decrypt, sign or verify the signature of a text in any web page, using GnuPG.
It will support some webmails. Right now, only GMail is supported, some useful buttons are added in the interface of this webmail !
FireGPG can sign/ecrypt basically anything in a HTML textarea and can verify anything on screen. However, during my test, if urls in the email was converted to hyperlink (thus, changed from original message), it couldnt verify it. Maybe they should create a function to trim the links or something.