The error "NS_ERROR_FILE_UNRECOGNIZED_PATH" comes from Firefox. It means that Firefox was unable to open a file:// URI and this most likely means that WWW::Mechanize::Firefox::get_local is broken on Cygwin. This most likely stems from the fact that Cygwin has its own, incompatible idea of file paths which needs to be translated to other Windows programs.

Maybe you can get around this by modifying get_local and eliminating the /cygdrive/c/ from the start of each file path.

I don't have Cygwin and see no particular need to work on support, as there are Strawberry Perl and ActiveState Perl on Windows. They provide similar support without the hassles of interoperatbility. If you submit patches that fix this, I will take a look at incorporating them into WWW::Mechanize::Firefox.