@utopiah with Tridactyl - no. there's a branch for the extension and native messenger that enables it but I chickened out because it was a bit scary (not only could any process on the machine control Firefox, but anywhere you forwarded the port to). maybe i will work on it again some day because the scary stuff is usually the coolest.

with just Firefox you can use https://developer.mozilla.org/en-US/docs/Web/WebDriver but they don't want you using it all the time. it makes the URL bar red etc.