As you know, the new extension platform that all browsers are switching to, Manifest V3, is supposed to disable ad blockers. Users don’t like this fact, to put it mildly, so companies are reluctant to abandon Manifest V2. For example, Google last December delayed the release of Chrome Manifest V3.
And now Mozilla has released a new one, 109- the version of Firefox in which Manifest v3 is enabled by default. Post on the company’s official blog explains how the new extension system will work, which should simplify their simultaneous development for Chrome and Firefox. So, while its implementation in Firefox is largely cross-compatible, the company says its version differs slightly from Google’s “in some critical areas,” mainly in security and privacy.
Firefox Manifest V3 continues to support the “webRequest” API to block entire categories of HTTP requests — the same mechanism used by most modern ad blockers. As a reminder, Chrome Manifest V3 instead requires developers to switch to “declarativeNetRequest”, which forces the use of a block list of certain addresses, which actually makes it impossible for them to work effectively.
This decision by Mozilla has its downsides, primarily that leaving this feature could pose a security threat — it seems that the company decided to take such a risk in order to preserve the ability to block content that it considers “one of the most important use cases for extensions”.