This is the link you need,
iPhone3,3_4.2.8_8E401_Restore.ipsw
I suggest using snowbreeze to jailbreak your phone. You can get it from here,
sn0wbreeze-v2.7r2.zip
You will need to unzip snowbreeze DO NOT unzip the ipsw from the first link. Make sure your iTunes is also up to date.
Open snowbreeze and show it the ipsw from the first link. Wait until its verified then select expert mode, then build ipsw and finally general. You custom ipsw will now be built. It will take a while so be patient.
Once that's all finished get your phone into DFU. Snowbreeze will show you how after its built your ipsw. Now open up iTunes it should detect your phone in recovery mode. Hold shift and press restore. Once iTunes is done your phone will be jailbroken...