I'm not sure for Firefox, but for Safari you could do the following:
- Install the XCode developer tools
- Make a copy of Safari.app
- Go into the app folder by right clicking Safari.app and choosing "Get package contents"
- Navigate to mainmenu.nib and double click it to open it in Interface Builder.
- Delete items from the menu that you don't want the users to be able to access.
- Test out your newly modified Safari.app, then ...
- Deploy it to the users.
There is probably a better way to do it, but I'm pretty sure that this would work.