In Firefly, white areas in a transparency map are opaque, and black areas in a transparency map are transparent.

You can use the same transparency map in the Physical Surface node. Just set the mode to "Opacity" instead.

Following screen cap shows a closeup of the trans map used for this fabric. Black areas are transparent, white areas are opaque (same type of map that you would use in FireFly). Just use your old typical Image Map node that you used for Firefly. You don't have to use a Cycles image node here.

The transparency image map node is then hooked in to the "Transparency" input of the Physical Surface node. Then the Transparency Mode is set to "Opacity." You don't need to flip it with an invert node.