Personally I have been using Komodo for quite some time. I did buy their commercial version as it has a nice runtime debugger. I like it as it is cross platform (windoze/mac/linux) and has a long list of supported languages. I first started using it years ago when it was one of the few that supported XUL for a Firefox plugin I was building for a customer.

There is an open source version http://www.activestate.com/komodo-edit and as I stated, the license was well worth the investment for my needs.

Good luck in finding one, I've changed editors a few times over the years, and it's painful! I like to have a nice IDE I can stick with, for 3 or 4 years now it's been Komodo.

On another note, it's built on the Mozilla engine and supports plug-ins just like firefox et all