I believe and after testing it by backing up using Chronus, we shall find several files for examples, after you back up the Contacts, you shall get
the following files in the sub-directory of your Iphone:
/var/mobile/Library/Preferences/TimeCapsule/ (several files

AddressBook.sqlitedb; AddressBookImages.sqlitedb; descriptions.plist
Similarly, if you back up your SMS you shall get the following files in the sub-directory of your Iphone:
/var/mobile/Library/Preferences/TimeCapsule/ (several files

:
sms.db; descriptions.plist
Now, you just copy all these files into your PC Subdirectory.
After you restore the firmware, you can SSH back these files to your iphone for the Contacts at the following subdirectory:
/private/var/mobile/Library/AddressBook
(replacing the created ones from the upgraded firmware)
for the SMS to the following subdirectory:
/private/var/mobile/Library/SMS
(replacing the created ones from the upgraded firmware)
You do not need to ssh the descriptions.plist to the respective directory
then just Reboot your Iphone, you will get back your contacts, sms, and anything that you back up and replace back to its respective application directory.