Adobe Flash Player can no longer access network / mapped drives

Firefox v48.0

Windows 7 x64

Flash Version: 22,0,0,209

After updates today I can no longer access all of our data on network drives (mounted) - I tried adding all kinds of trusted paths to the Global Security Settings Panel, but thus far unsuccessfully so.

I can enter the paths manually, but even here, when I try to access a drive's tree structure, I get blocked before I can even do anything - the drives appear as offline.

The issue became apparent using the Joomla extension Phoca Gallery, specifically the flash-based multi-file uploader.

I just found out the issue arises in Firefox, but not Chrome. I hear there are differences in how Flash is implemented in different browsers, I'm not sure if this might be the case here, or if there's simply something wrong with my browser set-up after the updates today.

Any help on how to get it working (zeroing in on the issue) very much appreciated.