Ok a lot of people worry about 3.9FB. Don't panic if you don't downgrade your phone to 1.0.2/1.1.1...
If you want upgrade back iPhone BL3.9FB to 4.6Orig download this:
Command: iphone-tools -B
More info here: http://www.hackint0sh.org/forum/showthread.php?t=33050
And one more option here: http://www.hackint0sh.org/forum/showthread.php?t=32668