I've a very strange issue with a 17" Powerbook 5.7. (A1107). This PB is a "hybrid" with PB 5.9 (A1139). Its motherboard is from A1107, but has a 1680x1050 display (instead of regular 1440x900).
Try to play YT-videos gives only the first picture of the movie, but then it does'nt start to play it! Nothing more happens!

I use exact(!) the same config as on my second PB, which is a 15" PB 5,6. (A1106). On this PB there are no issues and YT-videos a played perfect.
I copied complete Wayfarer-directory and put cgxvideo.library to Mossys:libs - so there is no difference in installation between this two PBs.
Both are runing MOS 3.15 and have PPC 7447A/1.67GHz-CPUs with Altivec.

Maybe interresting:
I know that older MOS-versions do have driver-problems (Radeon) with this PB, so that a "special driver" was needed at that time (thanks to MOS-Team). Later versions of MOS does'nt have that issues any more.....can't remember when it switched - think it was MOS 3.10.........