is tiberian sun free?

Yes. Yes, it is. You could just have checked wikipedia... it generally has such information, and links to reliable sources that confirm it.

"As of February 12, 2010, Electronic Arts licensed Tiberian Sun and its expansion pack Firestorm as freeware."

The original pack released by EA was basically just a zipped version of the folder in The First Decade, and a .reg file to give the game its required registry information, but since that counted on being extracted in "C:\Program Files\The First Decade\" it has since been repacked by community members into a better installer.

I suggest you take the one at cnc-comm.com. Though, the webmaster, Tore, has a better version of the pack in beta, which isn't on the downloads page yet. You might want to search the cnc-comm forum a bit; you'll be sure to find a link to it.