How to try Manifest v3 extensions in Firefox (developer preview)

I previously wrote that Mozilla still has no clear timeline when Manifest v3 will get mainstream support in Firefox. But it does not mean you can’t start experimenting and preparing for the inevitable MV3 migration in the Firefox browser right now.

Manifest v3 support was added in Firefox 101 release. Currently, you can only install temporary MV3 extensions after enabling several feature flags. For permanent installation you will need to use Nightly or Developer release channels.

Open about:config and set feature flags bellow:

• Set extensions.manifestV3.enabled to true
• Set xpinstall.signatures.required to false
• Set extensions.eventPages.enabled to true (this switches background scripts to non-persistent web worker)

Restart Firefox browser and navigate to about:debugging. Press This Firefox and Load Temporary Add-on. That’s it. Now you can start experimenting with MV3 APIs in Firefox and migrating your extension.

There are minor differences between how the MV3 manifest.json file is defined in Chromium and Firefox browsers. You can read all about them in this Manifest V3 migration guide from the Firefox team.