There are websites that break or don't work as expected with an AdBlock plugin. We had to redesign the section of our site that talked about advertising because various AdBlock plugins blocked chunks of the page or disabled the javascript for the contact form. This was very confusing to users even though they presumably made a choice to install an ad blocking plugin.

The specifics of how Firefox has implemented this feature are fairly conservative so that's less likely, but I think you need to be really careful about blocking certain lists of domains by default.