But...!
But people who actively block ads would never click them anyway, trust me! That's why they are blocked to begin with! So you're not loosing out (well maybe not if you get paid for the ads just being displayed and not actually clicked on)
My sister doesn't even know what Firefox is let alone know how to install something, she'll look at your ads I'm sure, as for me, I'm happy not to thanks. Maybe I *might* support certain sites in some other way as I sometimes do (and independent content creators) but advertising isn't the way for me.
There's nothing ethically wrong with blocking ads I don't think either. You choose to pump your site out to the world, what happens to it when it gets inside my computer is my business. You could always find some way to detect I'm blocking ads and bar me from accessing the site. The one site I've seen that being done on before I never visited again even with add blocking disabled!
Everyone won't block ads, don't worry, that will never happen. If Firefox came with adblock as standard there would be outrage and web-masters would throw up error messages on their sites telling users "This site can only be viewed with Internet Explorer Version 6.0 and higher!!!!" <--LOLZ