This way Firebug can help users to easily migrate into built-in Firefox developer tools – in case it’s running in multi-process enabled browser.
When opened in a multi-process Firefox, Firebug’s tools cannot be used.
Firebug 2 doesn’t work in multi-process browsers (i.e.
e10s) and converting it is too complex, so it will stop working when e10s is activated in Firefox.
You will be able to disable e10s for a while to keep using Firebug 2, but this won’t last forever.
Firebug 2.0.18 is compatible with Firefox 30 – 52 Firebug 2.0.18 fixes issue 8030.The extension is marked as multi-process (e10s) compatible so, it isn’t blacklisted and users can continue installing it.