mchung, one of our hard-working Firebug users, just uncovered a problem that I think hit many users:
If you enter Firefox’s Private Browsing Mode, then Firebug 1.4 will not stay open on reload or tab switching.
(See Issue 2011: Firebug console does not stay open when navigating between tabs).
Unfortunately when people report these symptoms, I have been telling them they have to create a new Firefox profile to fix it. And it does fix it, for a while. Until they once again set private browsing mode.
The only immediate fix here will be a warning that part of Firebug fails if you are in private browsing mode. The workaround is simple: don’t use Firebug in Private Browsing Mode.