It looks like the jssh plugin (required to run WatiN) still needs work going by FireFox Bug 586869, to quote:
Probably obvious to anyone who ought to be trying to maintain jssh, but, to get it back all you have to do is "hg up -r 3ca5d6674feb" and copy it to wherever you're going to work on it.
Haven't found much recent activity on SO for jssh either.
Firefox + jssh build
WatiN - Support for firefox and chrome
check the links - watin works with firefox latest version after installing mozrepl-jsshv2.6.xpi provided by Fabian
Thanks to Fabian. I hope the Watin and Firewatir community embeds this xpi in their repo