Firefox Extension Manifest V3 vs V2: What Actually Changed

If you've been following Chrome's controversial Manifest V3 migration, you might be wondering: does Firefox have the same MV3? The answer is: yes and no. Firefox has its own implementation of Manifest V3, and it's meaningfully different from Chrome's.

The Short Version

Firefox MV3 supports both MV2 and MV3 extensions

Firefox's MV3 is less restrictive than Chrome's

Firefox still supports browser.webRequest blocking in MV3

Service workers replace background pages (same as Chrome)

What Changed: Background Scripts

MV2 (background page):

// manifest.json { "manifest_version" : 2 , "background" : { "scripts" : [ "background.js" ], "persistent" : false } }

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MV3 (service worker):

// manifest.json { "manifest_version" : 3 , "background" : { "service_worker" : "background.js" } }

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The key implication: service workers don't persist state in memory .…