this happens to me too, where something (I haven't isolated what)
causes a situation where I can view the CSS tab and disable particular
attributes like usual, but cannot edit or add, or delete. only
restarting firebug doesn't make a difference, I have to then close out
of and restart Firefox entirely for it to behave normally again. it's
only started happened recently, and it's happened on several
occasions.
My environment is XP Pro 64 / Firefox (Portable) 3.0.4 / Firebug
1.4.2.
Andrew