Open up TinyUmbrella and look for your device. If it's not there, manually find the ECID. For Windows, go into Device Manager, then look for the Apple hardware, right click, properties. Then go into the Details tab, and find "Device Instance Path" in the drop down. Then there should be a row of characters. Copy them all out into Notepad or a Word Processing program. Copy ONLY the portion that has a bunch of 0's in it. It should be something like 000000XXXXXXXXXXXXX. Then there should be a symbol following it. Stop copying before you hit the symbol. That's your ECID. Open up TinyUmbrella, copy it into the Manual ECID field and click Manual ECID.
On the left hand tab, you should see a device under "Recent" with a ? and some characters. Click on that, and click on Save SHSH. Then wait until the loading bar on the bottom is done, then check under General and see if you see any 5.X.X iOS SHSH blobs. Should be something like iPhone3GS 5.X.X (#build). That's what you need to downgrade.