:clap2:
Any changes made in the tools>preferences will be applied globally but any changes in "edit site preferences" are applied to that site only and the global rules will continue to be applied everywhere else. So in this case, the mask as Firfox applies only to that site and any others where you make a similar tweak.
This is one of the features that make Opera so flexible and by adjusting these preferences on a site by site basis ( if required ) makes it possible to read all but a handful of sites.
Ok, a bit of a fiddle at the time, but once these site preferences are set......they remain so until and if you want to change them.
The only sites where such tweaking will have no effect are the ones authored by Microsoft, such as Hotmail, as Microsoft have made the decision that only IE, FF,and Safari will work for them.