diff --git a/package-lock.json b/package-lock.json
index dd429cc7..813bff86 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1810,9 +1810,9 @@
       "dev": true
     },
     "deepmerge": {
-      "version": "3.2.1",
-      "resolved": "https://registry.npmjs.org/deepmerge/-/deepmerge-3.2.1.tgz",
-      "integrity": "sha512-+hbDSzTqEW0fWgnlKksg7XAOtT+ddZS5lHZJ6f6MdixRs9wQy+50fm1uUCVb1IkvjLUYX/SfFO021ZNwriURTw==",
+      "version": "4.2.2",
+      "resolved": "https://registry.npmjs.org/deepmerge/-/deepmerge-4.2.2.tgz",
+      "integrity": "sha512-FJ3UgI4gIl+PHZm53knsuSFpE+nESMr7M4v9QcgB7S63Kj/6WqMiFQJpBBYz1Pt+66bZpP3Q7Lye0Oo9MPKEdg==",
       "dev": true
     },
     "default-require-extensions": {
diff --git a/package.json b/package.json
index b3ea23cb..e3cd65e1 100644
--- a/package.json
+++ b/package.json
@@ -30,7 +30,7 @@
     "archiver-promise": "~1.0",
     "chalk": "~2.4",
     "conventional-changelog-moulded-angular": "github:matatk/conventional-changelog-moulded-angular#0.2.0",
-    "deepmerge": "~3.2",
+    "deepmerge": "~4.2",
     "eslint": "~5.16",
     "fs-extra": "~7.0",
     "husky": "~1.3",
diff --git a/scripts/build.js b/scripts/build.js
index f2270bd9..91b23934 100644
--- a/scripts/build.js
+++ b/scripts/build.js
@@ -367,23 +367,16 @@ function mergeManifest(browser) {
 	const commonJson = require(common)
 	const extraJson = require(extra)
 
-	// For pre-2.0.0 deepmerge behaviour...
-	// https://github.com/KyleAMathews/deepmerge#examples (and scroll a bit)
-	const emptyTarget = value => Array.isArray(value) ? [] : {}
-	const clone = (value, options) => merge(emptyTarget(value), value, options)
-
-	function legacyArrayMerge(target, source, options) {
+	function combineMerge(target, source, options) {
 		const destination = target.slice()
 
-		source.forEach(function(e, i) {
-			if (typeof destination[i] === 'undefined') {
-				const cloneRequested = options.clone !== false
-				const shouldClone = cloneRequested && options.isMergeableObject(e)
-				destination[i] = shouldClone ? clone(e, options) : e
-			} else if (options.isMergeableObject(e)) {
-				destination[i] = merge(target[i], e, options)
-			} else if (target.indexOf(e) === -1) {
-				destination.push(e)
+		source.forEach((item, index) => {
+			if (typeof destination[index] === 'undefined') {
+				destination[index] = options.cloneUnlessOtherwiseSpecified(item, options)
+			} else if (options.isMergeableObject(item)) {
+				destination[index] = merge(target[index], item, options)
+			} else if (target.indexOf(item) === -1) {
+				destination.push(item)
 			}
 		})
 		return destination
@@ -391,7 +384,7 @@ function mergeManifest(browser) {
 
 	// Merging this way 'round just happens to make it so that, when merging
 	// the arrays of scripts to include, the compatibility one comes first.
-	const merged = merge(extraJson, commonJson, { arrayMerge: legacyArrayMerge })
+	const merged = merge(extraJson, commonJson, { arrayMerge: combineMerge })
 	merged.version = extVersion
 	fs.writeFileSync(
 		path.join(pathToBuild(browser), 'manifest.json'),