It didn't like me clicking around in the dev tools trying to figure out how it works. I can't reproduce it now, so I suppose it was a freak bug.

Oh, the dev tools?!

Yes, there's something not OK in FF in this regard. As I've clicked around in the inspector my fan went on and FF did hang for one or two seconds.

My best guess would be that it doesn't like such a "long line" (as it shows all the chars without line breaks in the inspector).

Also last year FF crashed once on my box while using the dev tools. I was shocked as usually nothing crashes on this box, ever.

So they have likely some severe bugs left in the dev tools, I guess.