This problem is almost always because you need the
libdvdcss package. This is no longer included in most distos because of legal problems with regionalisation of DVD's.
Go to url:
http://download.videolan.org/pub/libdvdcss/1.2.9/rpm/
Right-click the second package
libdvdcss2-1.1.9-1.i386.rpm and download it to somewhere on your hard drive. The find the downloaded file, right-click and select "Install Software". The YUM installer will run and sort it out for you. It will tell you if it needs any dependencies. Mostly I've found it finds them itself.
Once it's installed you should find DVD's will play.
I use now Kaffeine for all my multi-media. Though Xine also does a good job.
I've found that MPlayer does the best plug-in for Firefox. It even plays the video's on CNN.