Hi,
since some time Firebug it is very slow and after closing it I have to also shut down Firefox because it's very, very slow and nearly impossible to work with it.
Perhaps someone can use this comment.
My environment:
Firefox 40.0.3 (but problem was also in some last versions)
Linux 3.11.10-29-desktop
openSUSE 13.1 (Bottle) (i586)
KDE 4.11.5