I've never used seamonkey, nor do I have it installed locally (yet...)
-- however, I'd be willing to add support if it didn't require much
change to the extension's code.
You can force compatibility either using the Nightly Tester Tools[1]
(might require it to already be installed, though), or by unzipping
the xpi file, changing the install.rdf to include seamonkey support,
re-zipping the file into a new xpi, and installing that. If the
extension "just works" after you've tried that, then great.
It's strange that it says "Firefox 2.7" like that, but I believe
that's an issue with the application, not the extension.
[1]
https://addons.mozilla.org/en-US/firefox/addon/nightly-tester-tools/