Weird Firefox crash

This is just bizarre. I've received a new laptop at work and so I'm installing all my stuff. Firebug and YSlow have become absolutely critical to my day-to-day work, so that was on the list of extensions I want. Even searching for "firebug" locks up Firefox. Going directly to the site locks up Firefox. WTF?

Is, perhaps, the annoying virus scanner that PCs here come with doing something weird? This happens whether I use the corporate proxy or bypass it.

Anyone else seen this behaviour?