Of course, I don't know the actual percent of FF users that use ad block. But I think it's far more likely that it is a majority of current FF users, rather than it being a negligible minority. I think 2-3% of web users is not an implausible approximation of how many people use ad block overall on the web. It's not an obscure technology, it's quite well known, even if few people bother with it.

Edit: actually I'm way off - it seems estimates are typically around 30-40% of overall users on the web having some kind of ad blocker. So, the Firefox percentage being 60-80+% seems almost a given to me.