Firebug also causes a large performance hit, even for sites you haven't enabled it on. And it leaks memory, see https://bugzilla.mozilla.org/show_bug.cgi?id=66

Firebug also causes a large performance hit, even for sites you haven't enabled it on. And it leaks memory, see https://bugzilla.mozilla.org/show_bug.cgi?id=669730 https://bugzilla.mozilla.org/show_bug.cgi?id=669730. I shudder to think how many people have concluded that Firefox is slow and memory-hungry when it was really Firebug that was the cause.