Can you have a look in Device Manager and let us know if your DVD drive is listed with a yellow question mark or exclamation mark against it, and if there are any error messages against it.
Have a look here for instructions on how to get to Device Manager
Open Device Manager - Windows Vista Help
This is a Device Manager screenshot, on your machine it will probably be a DVD drive, with an error message next to it.
I would uninstall Firefox and reinstall it to attempt to repair any problems.
Let us know how you get on.