Here is just one example I stumbled across in the CSS for a page on WebDirect
background-image:-webkit-linear-gradient(top,#fff 0,#e8e8e8 95%,#cacaca 100%);
background-image:-moz-linear-gradient(top,#fff 0,#e8e8e8 95%,#cacaca 100%);
background-image:-ms-linear-gradient(top,#fff 0,#e8e8e8 95%,#cacaca 100%);
Note the "-moz-linear-gradient", a style specifically for rendering on Firefox browsers. (-webkit handles Safari and Chrome, -ms and the below filter: line handle different versions of IE)
I know Firefox is not supported at the moment, but I can't imagine they would have wasted the effort to add Mozilla specific styles to the themes if support wasn't intended at least at some point in the future.
At first I thought maybe this was just carry-over from v12 when Firefox was supported, but then I remembered that themes weren't even supported in IWP, so that can't be the case.
Firefox isn't supported, but that doesn't mean that webdirect will NOT work in firefox.
My guess is the CSS was copied in from a pre-existing package, of which I'm sure many were used in the creation of webdirect. I wouldn't read too much into it.