Adblock Plus is something akin to a TiVo for Web-browsing. Users who install the extension will find that their Web experience is radically changed--in that the vast majority of graphical Web advertisements will no longer be displayed within the Web-pages that they visit.
For those of you with short memories, it's worth noting that before TiVo was the only major game in town, there used to be another TV advertisement skipping technology. ReplayTV was vastly superior to the TiVo, in that it completely skipped commercials, instead of permitting users to fast-forward. Following a similar tactic to that was used by the major media companies (who had previously gone after Napster and the VCR), the TV networks essentially sued ReplayTV out of existence. The moral of the story: companies that have built their business models on advertising revenue do not take kindly to others who permit customers to skip those advertisements.
With that little walk down memory lane over, let us focus on the issue at hand--Web advertisement skipping technology. Essentially, it boils down to this: Web site designers depend upon advertising revenue to pay their bandwidth bills as well as to pay for the staff time that goes into making a successful site. Users do not particularly want to see advertisements, but except in a few cases where advertisements are extremely annoying, will for the most part put up with the ads in order to view the Web content that they are seeking.
There is a pretty big difference between the TV and Web site business models. A broadcast TV network, by and large, has fixed costs, no matter how many customers actually tune into the show. The same amount of electricity will flow to the TV transmitter, and the satellites above will still beam down the same number of 1s and 0s. Internet content is different, as each person's computer makes an individual connection to the remote server hosting whatever Web content the user is seeking. Each time users visit a Web site, the server consumes bandwidth to send the content of the Web page back to the user--and that bandwidth costs money.
Thus, every time someone uses advertisement-blocking software to avoid the graphical ads embedded within a Web site, they are denying the Web site operator revenue that would otherwise have gone to pay for the bandwidth that is consumed during that browsing session. While it could be said that TiVo users are freeloading from the broadcast networks, users of Web advertising skipping technology are far closer to theft than they are to freeloading. This is not a clearly defined issue, but there are a significant number of moral issues at play.
Which now brings us to the technical issues involved in this particular story...
The person running the anti-Adblock Plus campaign has been unable to remotely detect which Web surfers visiting his site have installed the extension, and so, in an effort to pressure both the developers of Adblock Plus and the Firefox browser development team, has instead called for Webmasters to completely block the Firefox browser. Call it collateral damage, if you will.
In the end, a few things are clear: Users of advertisement-skipping technology are essentially engaged in theft of resources. Web site owners have not, yet, wrapped their Web sites in shrinkwrap contracts, and so while the ad-skipping may be immoral, it certainly isn't illegal. Web site owners are perfectly within their rights to utilize any and all browser/extension/Web behavior detection technologies in order to blacklist the ad blockers. Similarly, creative users are more than within their rights to evade whatever detection technology the Web site designers use.
The real question to be answered is: will other Web site owners wish to get themselves into an arms race that they almost certainly cannot win?