Rated 4 out of 5 stars

http://xsidebar.mozdev.org/modifiedmisc.html#copyplaintext
copy_plain_text-0.3.3.1-mod
Works with SeaMonkey 1.0 - 2.1.*, Firefox 2.0 - 3.7.*, Thunderbird 2.0 - 3.1.*
For latest versions of browsers edit maxVersion in install.rdf