"The startup is Tyba and it’s a simple concept: You download a Firefox extension which allows you to rate web pages and append tags (a feature I like is that when you come from Google or Yahoo the search term is automatically sent with your rating). The rating and tagging is simple, done by clicking stars or using keyboard shortcuts. Then when you feel like it you can login in to the web interface and organize the links you’ve rated into groups. These groups become public and anyone can view them. The other Tyba users can follow your groups and you in turn can also follow other people’s groups. When you follow a group you can do two things, see when new links are added to them and you can search through them. It’s a bit like building your own search engine with the links you rate and the groups other people make. Right now all this functionality is there in its most basic form."
I've developed a few Firefox extensions, though I'm not available to help as I'm driving my own company.
What kind of platform-specific code does the extension use that makes it difficult to run cross-platform? I'm surprised it has problems because no extension I've ever coded required much more than visual tweaks across Mac/Windows/Linux. Some extensions clearly are tied to a platform, but from what is described here, I'm not seeing a piece that can't just be XUL/JS.
Good luck, regardless!