For quite a long time the highest custom PS3 firmware version remained 3.55, recent developments allowed to jailbreak higher firmware versions,
which proved to be safe to update.
While you are still on 3.55 though, it is highly recommended to enable QA Flag and dump NAND/NOR. These simple procedures could save you a lot of trouble and help recover from brick in case anything goes wrong. Then download a jailbroken CFW. I recommend to use Rebug releases, in this case, version 4.30.1. When download completes, take a thumb drive, create a PS3 folder on it, UPDATE folder inside, and copy the downloaded .pup file there. Rename the file to ps3updat.pup if required. Now plug the thumdrive into Playstation 3 USB port, select System Update from the menu and choose Update from storage media. If you placed the update file to the right location, the console will see it. Continue installation, accept the terms and tick the automatic shutdown option, just in case.Click start.
After the process is completed, the console will automatically shut down. Power it on. Let the warning that hardcore gamers may produce lunatic kids sink in. It’s done. You’re now on 4.30, but still keeping all the jailbreak goodies like package installation and game folder mount. And as this is Rebug, and you were keen enough to enable QA flag while still on the old firmware, you also keep access to debug settings.