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.

https://bugzilla.mozilla.org/show_bug.cgi?id=815603

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.