Last week web browser maker Mozilla announced that it would stop selling ads in its Firefox browser. This week the company rolled out an app called Focus for Apple's iPhones and iPads that blocks companies from tracking people's browsing in Apple's Safari browser but also blocks ads.

In an interview with Ad Age, Mozilla Chief Legal and Business Officer Denelle Dixon-Thayer explained why Mozilla created the ad-blocking app, how the app rewards sites that respect browsers' Do Not Track settings and whether the company still considers itself to be in the advertising business.

Ad Age: Why did Mozilla decide to create an app that would let iPhone owners block content in the Safari web browser, a web browser that competes for market share with Mozilla's own Firefox web browser?

Denelle Dixon-Thayer: There were two reasons for it, at least that I can think of right now. The first one is the fact that [the latest version of Apple's mobile operating system] iOS9 made this content blocking API available, and so it made it an option. The second thing is it's very consistent with respect to what we've done in tracking protection in the private browsing mode [in Mozilla's Firefox web browser], given that we're trying to focus on the issue that users have indicated as a problem, which is the tracking. So with the availability of the content blocking API in iOS9, we saw it as a very consistent step for us. We would have liked to utilize it even in our [Firefox] browser on iOS, but that's not something we can do. The API doesn't allow us to do that.

Ad Age: Focus is described as a "content blocker." That's Apple's nomenclature. What's the full scope of content that Focus will block?

Ms. Dixon-Thayer: We're using Disconnect's list, and this is focused on tracking protection. It's the same list that we use in our tracking protection in private browsing. So we're focused on the problem of trackers and not any particular piece of content or area of content. The list is actually a public list. It's available to see, so we were very consistent with respect to our product that we already had on desktop.

Ad Age: I understand the main purpose of Focus is to block ways that people can be tracked through a mobile web browser, but it also has the effect of blocking ads. Was there a decision made to block both tracking and advertising, or is the ad blocking a side effect of the tracker blocking?

Ms. Dixon-Thayer: The focus was on tracking protection. Definitely. We are approaching this very much from content neutrality. It's how we've approached many things with respect to Firefox. We don't look at advertising, for example, as the problem. It's the tracking that the users had identified as the problem. So for us, we started with where the problem sits and moved to a solution to the problem, rather than identifying content as an issue

Ad Age: I noticed that when I had Focus enabled, I didn't see any ads on Google's search results pages or on The New York Times' site or on Ad Age's site. But I did see ads on Yahoo's home page and on Yahoo's search results pages. Those are specific examples. But are there any specific considerations around which ads or which sites are blocked and which aren't?

Ms. Dixon-Thayer: No, that's why we use the public lists. So it's very clear who's on the public lists and why they're on the public lists. That's something that we feel is really important, being able for folks on the publisher side to be able to see that they're on the list and to question why they're on the list and [that] they're not put in a permanent penalty box. So that there may be things they can do to get themselves removed from the list. The list is the tracking protection list provided by Disconnect. The one thing we have contemplated, and it's part of the way we would like for this to adapt, is that if you do respect Do Not Track, then you're indicating you're not going to track. That's a way to get off the list if you're on it.

Ad Age: That's something that's just being contemplated, not enacted yet?

Ms. Dixon-Thayer: I actually think that's part of what Disconnect's list already has in it. [A Mozilla spokesman later confirmed that ads on sites that respect Do Not Track requests are not blocked.]

Ad Age: I was looking at Disconnect's lists. There are some domains on Disconnect's public lists that appear on its blocked and unblocked lists. The ones that I noticed were a number of Google domains that appeared on both lists. Do you know why that is, and what involvement will Mozilla have in terms of which domains are included on which lists going forward?

Ms. Dixon-Thayer: I don't know the answer to that. That's something that Disconnect could probably answer better than I. [A Disconnect spokeswoman said that the company includes some domains on both lists so that a domain like analytics.google.com is blocked when identified on a third-party site like www.adage.com but not blocked when identified on a first-party site like www.google.com].

We want these lists to be as pure as they can be, so the involvement that we would have is just encouraging that kind of thing with respect to Disconnect. It's very important to us that the Focus app is not going to be something that we monetize, and we would like the list that we're using to be similarly situated with respect to that. So there's no way to buy your way on or off the list, whether it be a whitelist or a block list. That's important to us that that continues to be the structure. Other than that, in terms of the content and who's on and off, that's something that Disconnect at this point is responsible for.

Ad Age: In your blog post announcing the launch of Focus, you mentioned that you've based "a portion" of Focus on Disconnect's public lists of trackers to block and not to block. What else is the blocking based on?

Ms. Dixon-Thayer: I actually don't know what you're referencing. If we pulled up the blog post, it might help me.

Ad Age: Sure. In the third to last paragraph, the second sentence reads, "To do this, we've based a portion of our product on a list provided by our partner Disconnect under the General Public License."

Ms. Dixon-Thayer: I think the portion of our product, there are other indicators in there that we might expand the scope of this product to be performance-based as well, so that there could be some other capabilities of Focus. I think that's what that's referring to.

Ad Age: Could you elaborate on what those other indicators are and how you're thinking about expanding the scope?

Ms. Dixon-Thayer: If you focused on other problems that users see, so that you could do problems separate from tracking. If a user wanted to indicate that performance was a larger problem for them, there could be ways that we could improve the performance of users by having other lists or just having content throttled or something like that. There's also fonts that could take a long time to load. So focusing on those kinds of characteristics that decrease performance. Those are details that are still to be worked out, but there could be other areas that the app could go to.

Ad Age: Days before Focus was announced, Mozilla announced that it would stop selling ads in the Firefox web browser. Why was that decision made? And is there any relation between Mozilla's decision to stop selling ads in Firefox and then rolling out a content blocker that also has the effect of blocking ads?

Ms. Dixon-Thayer: No, there's no relation. The way that you submit applications into the App Store, we're sort of at the mercy of when those apps are approved. So there's no relationship at all with respect to that. The decision with respect to content services is focused on how we as an organization think it's important that we look at content discovery. We think advertising in Tiles [site thumbnails featured on Firefox's new tab pages that could be sponsored links] was actually successful. The engagement ratios were very high. So it's not looking at the work that was done and deciding that's not a successful area for us. In terms of what we focus on is being the agent for the user as they traverse the web content is an area we want to look at, and content discovery and how we can bring that to users. The work that we did with respect to advertising, the way that we demonstrated that it can be done differently, that it can be done while respecting the user, is important and it's crucial to understand this area and this space. But we want to, as an organization, focus more on the content discovery piece.

Ad Age: Does Mozilla still consider itself in the advertising business, or has Mozilla effectively exited the advertising business?

Ms. Dixon-Thayer: From my standpoint, all of us who believe that the web is an important place to be and we want publishers to be able to provide content to our users, we're all in the advertising business because that's the revenue ecosystem that powers the web. We are never going to step away from that being an issue that we believe is important to engage on and to try to improve for all us. We recognize that we're all stakeholders in a really important global asset.