There's no need to add reasons why you should be using Firebug while creating your web applications, specially if you are into DOM scripting. I'd just like to add a small detail that we may miss when using Firebug to search in our code: how to find the next result?

It's simple, just press Enter; for previous results use Shift + Enter.

Many of us are used to F3 and Shift + F3 in Firefox but may get a little confused when we start to search with Firebug.