The Next Web is reporting from an e-mail they received explaining that Google says that extensions developers can now start monetizing via Google Adsense, where Google state:

We are updating our ad policies to allow extensions to monetize through ads. Please ensure your extensions are in compliance by reviewing our extension ads policy guidelines.

Google seem to be giving developers the ability to put ads in extensions that feature more persistant visual elements, TNW have used an example by adding them to the TNW extension for Chrome:

But, it is nearly full circle for Google, because when extensions were first announced back in 2009, the early extensions focused on a few areas, one of which were to block ads. Now these same extensions can feature ads, making developers money on ad revenue. It has been noted though, over the course of the Extensions timeline, that some were being used to replace or display alternative ads on websites, which takes potential profits away from the site owner.

Google's Chrome Extensions Policies completely forbid this exact thing:

Ads associated with your app may not interfere with any ads on a third-party website or application. You may show ads alongside a third-party website only if all of the following criteria are met:

• This behavior is clearly disclosed to the user.
• There is clear attribution of the ads' source wherever those ads appear.
• The ads do not interfere with any native ads or functionality of the website.