Usually if you try to install an add-on packaged as a .xpi file, it contains a list of compatible Firefox versions and Firefox won't enable it unless you are running one of those versions. So... I think the question should be directed toward whoever packages the add-on. I searched a bit, but couldn't figure out who is in charge of that.

Random web results that might help:

SourceForge.net: WatiN: Detail: 3274683 - Add Firefox 4 support

watir - Alternatives to FireWatir working with Firefox 4 - Stack Overflow

Thanks, jscher2000. I guess the real question is: Does anyone know if there is such a thing as a jssh.xpi plugin for Firefox 4.0.1? If so, who makes it and where can I download it? Has anyone done any successful automation testing with Firefox 3.x and WatiN? - Just curious -

See also /questions/820677

I appreciate the reference, cor-el. I am actually following that entry as well. I did go back to Firefox 3.6.17 and installed jssh.xpi for version 3.6 and successfully ran an automation script using WatiN. It works pretty sweet. I am looking forward to using FireFox 4.0.1 but I don't have a clue where to get a similar jssh.xpi for it. Thanks again.