I think this is worth kicking off a discussion on here.
I currently use SharePoint Designer 2010 + FireFox + firebug and it is a bit heavy going for me to write (as editor is basic) but very quick to test ( using alerts or debugDiv.innerHtml = ‘what ever’). Now this was ok for a few functions but wI am starting to write losts of nested closures in ever more functions and object definitions. Hence, I could do with better tool support. Also, I tend to develop my JS in Style Library/scipt (content db) so that I can check in periodically. I wonder also if I really should have the js locally ( add file:// to master page) until the final checkin then this would allow me to use editors like Aptana or the JavaScript Parser addon for Vs2010 or Resharper.