Very annoyingly, in the FF 24 beta, Firebug basically crashes the browser every time you open it (or switch to another tab and then back). Not sure if it's FF's problem or Firebug's, but it's really annoying.
Interesting, you don't have lag when first opening it? I was investigating pretty much the same thing yesterday, in the end my problem was that the JSD was ridiculously slow to toggle, but firebug is already working to move to JSD2 so I can't really complain too much. There is a test extension you can use attached to this bug.
I was getting something like 100ms to pause and 10000ms to unpause. With 5 tabs loaded and a couple hundred tabs unloaded. I disabled the javascript parts of firebug for general use, which does avoid the problem most of the time.
Aha, you're right! Disabling the Script tab helps immensely. It's a shame since I need the Script tab for a lot of work, but it's not too much of a pain to disable/reenable.
Crashes and slowness that vary with Firefox versions is Firefox's fault, and in this case the note about tab switching points pretty clearly to a problem with the debugger service.