Which version of Firefox are you trying to download? The latest FF17-015?
Which product of ours are you working with that's failing the signature check?
I just double-checked that latest Firefox download with both Ivanti Patch for SCCM 2.3 and Patch for Windows Servers 9.3.4510, and both were able to verify the file, so it's like something environment-specific. I would recommend opening a support case to look into it further if there aren't any certificate issues or anything that jump out at you. We'd be happy to help diagnose the issue and get it resolved for you.