In order to have saved the 4.2.1 you had to do it any time before March 9.
Its not what is installed on the phone that matters but what blobs apple's servers are releasing.
When 4.3 comes out they stop releasing 4.2.1 blobs so Tinyumbrella cant retrieve and save those. Again shsh's are not saved on your phone and what firmware version is installed on your phone doesnt make a difference.