Chromium was set up to launch xdg-open on my system, which takes you to the correct app in Ubuntu Software Center.
Firefox should give you the opportunity to select a program to open. You'll need to navigate to /usr/bin/apturl and it should work.
Opera makes you go to Preferences -> Advanced -> Programs where you can add a protocol helper for apt. Simply type apt as the protocol and apturl as the program to get it to work.
I hope that helps.