There is a solution to use Firebug version 2.0.18 in the latest version of Firefox (this time 51.0.1).
To enable the old Firebug:
In the address bar, enter about:config and
find browser.tabs.remote.autostart. Set it to false.
Also set browser.tabs.remote.autostart.1
and browser.tabs.remote.autostart.2 to false.