/dev/fb is the frame buffer for graphics. It may not have picked up your graphics card drivers on install, in which case Google will be your friend.
It's sometimes caused by running on a read-only file system. If that's not the case, more information on fixing the error is here:
If other devices on the system are working correctly, the most likely reason for this error is that the SUNWdfb package was removed or never installed. Insert the installation CD-ROM, change to the Solaris_2.* directory, and run the following command to install the packages SUNWdfbh and SUNWdfb (for your machine architecture):
pkgadd -d .
If other devices on the system are not working correctly, the system might have a corrupt /devices directory. Halt the system and boot using the -r (reconfigure) option. The system will run fsck(1M) if the /devices file system is corrupted, most likely fixing the problem.