Welcome to RTM's Mega Jailbreak Thread!
Now, Firstly. You need to know what exactly a Jailbroken ps3 is.
Basically, A Jailbroken ps3 is where you are allowed to run unauthorised code which then allows you to install hombrew apps onto your Play-Station 3.
There are both advantages and disadvantages to having a Jailbroken Play-station 3.
I will name at-least 2 of each.
Advantages
You can run backed up games whether they are from your Play-station 3, Or a USB. You can back up games not matter what.
One other good thing about having a Jailbroken Console is that you can mod any game you have dreamed of doing.
Disadvantages
You can get a console ban if you get caught modding, That is why it is recommend to have have 2 Play-Station 3's.
You can brick your Play-Station 3 while in the process of Jailbreaking it.
Downgrading
Can I downgrade my Play-Station 3 without a E3 flasher. No, You can not unless you're already on some kind of Custom Firmware. In that case you would use Rogero's 999 Downgrader.
You can see if your console is downgradable by downlaoding this.
http://www.mediafire.com/download/l0061hcdavwmsb9/MinVerChk.rar
Basically, when you download it You will see a PS3UPDAT.PUP file. This is an update, It will basically tell you if your Play-Station 3 has ever been on 3.55. If yes, You can downrgade it with an E3 flasher. If not, You can not jailbreak that console at all.
You can purchase a E3 flasher here.
www.modchipcentral.com
If you want to Downgrade with an E3 flasher yourself. You can look at the video below.
I will carry this on When I have more time.
Any mistakes, Feel free to edit it.