See this:
http://kb.mozillazine.org/Problems_printing_web_pages

When my wife updated Firefox to version 7.0.1 she was no longer able to print items from the web such as coupons. She can print from her email (Cox) account. She is running Vista. When I updated to Firefox 7.0.1, I did not have a problem. I am running Vista also. Tried using the Firefox help page but no help so far. I did notice that on her computer in the "Plugins" for Firefox that HP Smart Web Printing is now disabled and is not compatible with Firefox 7.0.1. She had no problems until she did the update.