FireFox Web Debugging Tools

I normally work from home as a technical web designer anything PHP,MySQL,Perl,ColdFusion,CSS,JavaScript,XML etc etc and think myself pretty up to date, well for the past few weeks I've been working on a contract with another programmer Hanif and he's showed me the Error Console in FireFox which lists and indicates HTML,CSS and Javascript errors, when I said how useful I found that he then showed me the FireFox add-ons FireBug and web Development which I found sensational.

What I'm trying to say here is how useful it is to co-operate with other experts and also of course how wonderful Firefox is