Manifest V3 updates landed in Firefox 128

With the release of Firefox 128 comes additional Manifest V3 improvements as the engineering team continues to make cross-browser compatibility work a key focus, especially related to content scripts and the scripting API.

In Firefox 128, support is now available for the MAIN execution world for content scripts declared in the manifest.json file and scripting.executeScript, which allows extensions to inject a script in the web page execution environment. However, unlike using window.eval from an isolated content script, the script injected into the MAIN world is not blocked by a strict webpage CSP. Please be aware that content scripts executed in the MAIN world do not have access to any WebExtension APIs.

Developers should also take note that the non-standard Web API events overflow and underflow have been deprecated. Use of these events should be removed from extension documents before the release of Firefox 131. Extension developers can set the “layout.overflow-underflow.content.enabled_in_addons” about:config preference to false in Firefox Nightly to determine whether their extensions will break in the future.

And to make migration less burdensome for developers, we intentionally back ported MV3 APIs to MV2. This means extensions developers will be able to more easily transition their extensions to MV3 gradually. For example, in Firefox 128, despite the fact that the MAIN world and the match_origin_as_fallback features are only supported for MV3 in Chrome, we will still support them on Firefox for MV2.

Lastly, with the launch of the Android permissions UI in Firefox 128, we are able to facilitate the control of the optional permissions and supporting host permissions that landed last month in Firefox 127. This means we are now able to fully support MV3 on Firefox for Android.

For a list of all changes relevant for add-on developers available in Firefox 128, please see our release notes. For more information on adopting MV3 in general, check out our migration guide. If you have questions or comments on our Manifest V3 updates we would love to hear from you in the comments section below or if you prefer, drop us an email.