Search as I may I cannot find any option or setting to return to the previous setup.
This was caused by bug #8018 in Firebug:
Using Firebug 2.0.16 the Computed side panel lists all CSS properties under the Other group starting from Firefox 46.0.
It has been fixed in Firebug 2.0.17:
The Firebug team released Firebug 2.0.17. This is a maintenance release ensuring compatibility with latest Firefox releases.
The beta channel on AMO is also updated.
Firebug 2.0.17 is compatible with Firefox 30 – 49
Firebug 2.0.17 fixes the following issues: 8003, 8012, 8018, 8022
Install the latest Firebug to get back the old behaviour of the "Computed" tab.
I can confirm it is working as expected on Windows 7 64 bit, Firefox 47.0.