sorry mate now ive looked at this post you have another option i ve just downgraded an iphone 3gs from 6.1.3 to 5.1.1, so this should work for u if u have defo saved your shsh blobs for 5.1.1 i [upgrade or downgrade if you have your blobs saved]would do that fw as its the most stable
First 1.use ifaith - Connect your device and open ifath [remember if your on vista, win 7 or win 8, you right click the .exe file /properties/compatability tab/tick the option to run windows xp service pack 3 mode]
then choose the option ive circled
,
then this screen will appear
once your device shows up on the right select it , or you might get a prompt box saying see available blobs for your device , once you follow the on screen instructions you will have saved your shsh blobs , once these are saved you have to download the 5.1.1 ipsw file from
HERE.
Then you need sn0wbreeze [attached in the post], the idea now is to create a custom ipsw using you saved blobs stitched into the original 5.1.1 ipsw open sn0wbreeze up and follow the on screen instructions, if your blobs are wrong or the ipsw it will let you know, if you do this correct you will have a custom ipsw file that can be restored in itunes.
This will only work if you defo have your blobs saved for 5.1.1 or other fw yo want, if you need any help mate just ask