[aur-general] Delete firefox-firebug

[Kevin Vesga]

Please remove firefox-firebug[1] as it a duplicate of
firefox-extension-firebug-stable[2]
and its out-dated.

[1] https://aur.archlinux.org/packages.php?ID=50889
[2] https://aur.archlinux.org/packages.php?ID=44809