This had been bothering me on ie9 on my main computer, but not on my other one. After reading this thread and finding some of you having the problem with ff I realised it must be a settings issue and not a problem to be solved within OC. I have never had a problem with ff and my checkout, however after a bit of playing around I have the solution for ie9 at least which is to do with the compatibility view settings and can be fixed as follows : -
1. Within ie go to - Tools
2. Then select - Compatability View Settings
3. Then select - Display all websites in compatibility - by checking it from the options
4. Close
and voila it works!
Everybodies happy!
Once I I work out the ff solution which should be similar, I will let you know. I hope this helps.