Brave doesn’t only do ad blocking; it also handles tracking, cryptocurrency mining and fingerprinting scripts too.
In addition, browsers do not allow the kind of low-level access required to block everything Brave blocks.
From the recent Brendan Eich AMA: https://www.reddit.com/r/BATProject/comments/7l4033/transcri...
bat-brendaneich Admin 1:31 PM First question may be about the idea of a BAT extension for other browsers, but that is premature. The big problem with UGP grants and Gemini-phase ad revenue shares to users is fraud. Just user-funded contributions has a fraud problem too: as with buy widgets, stolen CC identity => $20 charge to buy BAT => contribution at scale via sybils/mturk-users/bots-with-enough-work => settlement to colluding but verified (small blog) publisher. That's why I mentioned sybil-resistance above. So we can't just make a wish and try monitoring Basic Attention Metrics from an extension, and attributing BAT flows and creating user wallets, from extensions. There can be other problems, which I've noted elsewhere: lack of extension APIs to do all we do for the BAT platform to work (block ads/trackers, HTTPS Everywhere, Fingerprinting Protection, BAM and the ledger), extensions run in JS sandboxes with API limits. So to put first things first, we will build in Brave while keeping our code as separable from chromium (or the mobile webview on iOS) as possible. After we have those endpoing and on-chain specs I mentioned in pretty good shape, we can assess extension feasibility on Mozilla, I can't speak for them. The friend who contacted after the BAT sale signaled interest but said it would take time, to which I said "same here" (per roadmap). I hope that answers the two reddit questions.