I can confrim that after trying everything else I could think of, the work-around of using AOL Desktop for Mac (I downloaded 1.7) works to print coupons.
I also sent an email to Coupons.com detailing the problem with Safari and Firefox (the only two browsers they list as being compatible with their software and Mac OS X) and letting them know about the work-around of using AOL Desktop for Mac, so it's not an incompatibility with OS X itself - just with their implementation with data from Safari and Firefox. I'd suggest everyone else do the same. Here's the link to their "Contact Us" page:
http://www.couponsinc.com/corporate/ContactUs.aspx
I just used bogus info for the phone number (555-555-5555), company name, etc (I did give them one of the email address I use for junk mail and subscription lists that I only rarely check).
I also selected "Problems with or Questions about Printing & Using Coupons" as the option on the "Please contact me to discuss" drop down, and then "I cannot print coupons" on the "Problem" dropdown menu that appears after that.
Hope this might help others.
Oh, and here's a link for AOL Dekstop for Mac:
http://daol.aol.com/software/desktop-for-mac/