Properly supporting Windows and Mac OS X requires testing on the 3, maybe 4 latest OS releases. Properly supporting Linux requires testing on the last 3 or 4 versions of Ubuntu, Fedora, Mandriva, Mint, Arch, Gentoo, and who knows what else. Furthermore, while code that works on Windows XP is almost guaranteed to work on Windows 7, and code that works on OS X 10.6 is almost guaranteed to work on 10.8, there's no such guarantee on Linux.

All we build is a browser extension, and we've seen issues on Linux that depend on the version of gcc used to build Firefox, the version of d-bus, the window manager, and the desktop environment. I can't imagine what it would be like to support a real app.