++1 for HtmlUnit. I'm now writing selenium tests for a GWT app that I'll "play" on various browsers to verify correctness, but I'll run them on HtmlUnit as part of the post-build regression tests.

I can't find anything about a headless firefox on the jaxer site. I thought it was server-side javascript?

Jaxer is server side javascript, with access to a DOM. When I looked at it (more than a year ago now), they accomplished that via a headless firefox 3. I'd guess they still do that, since it worked for them.

Yeah, I just assumed it was a javascript interpreter with a DOM. (Obviously you don't need a whole browser for that.)

I wonder what the performance implications are. Firefox seems way too... single-threaded?

it sounds like Jaxer has a build of SpiderMonkey running inside it which provides server side JS support.