Since upgrading to Vista I have really missed the script debugger, it comes in extremely handy when working through your JS issues on IE. If you've never used it, you are missing out. It's quite a bit more powerful than the standard syntax checker that comes with Firefox, but I assume there are is a similar plug-in for Firefox.
To start, go download it here. You need to save it locally, perform the install, then update your IE settings under tools --> internet options --> advanced --> uncheck 'disable script debugging...'. Once that's done, you might want to play it safe and restart your pc. At least that's what I had to do.
I want to stress the fact that you need to save it locally, and then install it, as 'running' it from the microsoft site does not kick off the install properly.