-has more glitches (things doing plain weird or not working sometimes)
-requires "activation" of each panel and reloading of the page
-sometimes suddenly opens for no reason when you visit a webpage without the intention to debug it
-is impossible to use if you want to debug a project with hundreds of js files (e.g. uncompiled JS of a large project) because it has a bad type of dropdown for selecting the JS file
-generally just requires more clicks and more frustration to get things done
The one in Chrome:
-has a better console (easier to type commands, inspect variables, etc...)
-works immediately as soon as you open it (no "this panel was inactive during page load")
-has more features
Note that I was talking about Firebug here. Firefox also has its own dev tools, at least an "Inspect Element", with even a 3D view of a page allowing to see divs outside of it and so on, and that one is awesome!
I'm curious from your side (maybe also to learn a thing or two about Firebug): How would you compare it to Chrome? What features make Firebug stand out?
EDIT: Why do I need to add multiple newlines between the "-" to make Hacker News post them as newlines?
This is unfortunately true, and that's why it's exciting to see Mozilla do something about it. As it stands right now, Chrome is the better tool, and if you prefer Firebug to it chances are you haven't tried Chrome for long enough.