Firefox Extension Manifest V3 Migration: What Actually Changed

Manifest V3 (MV3) has been a contentious topic in browser extension development — mostly because of Chrome's aggressive changes to blocking WebRequest API. Firefox's approach is different and more developer-friendly. Here's what actually changed and what you need to know.

The Short Version for Firefox

Firefox's MV3 implementation is more permissive than Chrome's:

Blocking WebRequest still works (unlike Chrome, which deprecated it)

Background pages → service workers (same as Chrome)

Browser action → new unified action key

browser_action / page_action → deprecated, use action

What Actually Changed in manifest.json

Version declaration

// MV 2 { "manifest_version" : 2 }

// MV 3 { "manifest_version" : 3 }

Enter fullscreen mode

Exit fullscreen mode

Action key

// MV 2 { "browser_action" : { "default_popup" : "popup.html" , "default_icon" : "icon.png" } }

// MV 3 { "action" : { "default_popup" : "popup.html" , "default_icon" : "icon.png"…