All hail Firebug!
The sad thing is, I’ve never installed a Firefox plugin for web developers before. Yes, I know, I know. I’m kicking myself, believe you me.
I installed Firebug the other day, because my Firefox profile managed to hose itself (grrrr) and I had to reinstall a bunch of plugins anyhow, so why not pick up a few new ones?
This morning, I had one of those horrible moments when you click on a perfectly innocent link in your new design and the resulting page is oh-noes b0rked.
Firebug found the problem within thirty seconds of my figuring out how to use it (which itself took less than five minutes). Isolating it on my own would have taken me hours of cussing and fiddling.