I can't even count how many airbags I've replaced on Contours and Mystiques!
In anycase, to answer your questions, No you don't need to replace the sensors UNLESS they are physically damaged. You MAY need to replace the module. It is located in the console under the radio.
Just remove your old bags and instll the new ones. Don't forget to pull the fuse before installing the new bags(just in case). Also, put in the bags before you replace the windshield(again, just in case). On the passenger side you may need to do some rebending of the dash and new airbag in order to make everything fit together right. When that bag goes off it bends up the dash and ALWAYS busts the windshield. On the drivers side you will need to bend back the "spring loaded" mounting plates. When that bag goes it bends them rearward.
Once your bags are installed put the fuse back in and start the car. If your airbag light starts flashing, check the code, but it's a good bet that the internal fuse blew in the module. If that's the case then you need to replace the whole module. 9 out of 10 of the ones I've fixed have also needed the module.
I may have the parts you need to fix your Contour. If you're interested or have any more questions, let me know.
Good Luck!
