We need more information, please. What kind of phone do you have, and what software are you running on your phone (Firefox Home for iPhone, Fennec for Android, Firefox for Maemo, ...)?

For some possible solutions, see: Firefox Sync troubleshooting and tips

I also had this problem with a build from a few days ago.

I was using Fennec for Android nightly on a Motorola Droid 1, it seemed to successfully log in to Sync but never updated bookmarks. It did come up with tabs from my other computers, so Sync wasn't completely failing.

The most recent build I downloaded just doesn't open for me, but I guess when I have time I'll test with the most recent version I can find which works.