If your on 3.55 and you wish to stay at 3.55 then use the 3.55.3 REX pup. can be found at
http://rebug.me
If your on 3.55 and you want to go to 4.21 then use the 4.21 REX pup. can be found at
http://rebug.me
Now once you have downloaded the firmware that you wish you will need to rename it like this.
for 3.55
REBUG_3.55.3_REX_PS3UPDAT.PUP to PS3UPDAT.PUP
for 4.21
REBUG_4.21.1_REX_PS3UPDAT.PUP to PS3UPDAT.PUP
Now you will need a usb stick formated to fat 32, then you will need to create a folder on the root of the usb stick called "PS3" then inside that folder you will create another folder named "UPDATE". So it will look like this.
X:/PS3/UPDATE/PS3UPDAT.PUP
PS3UPDAT.PUP being the firmware that you are choosing to install.