diff --git a/src/env.js b/src/env.js
index 39154db..81cb6c6 100644
--- a/src/env.js
+++ b/src/env.js
@@ -31,7 +31,15 @@ const env = {
 	 * @static
 	 * @member {Boolean} module:utils/env~env#isEdge
 	 */
-	isEdge: isEdge( userAgent )
+	isEdge: isEdge( userAgent ),
+
+	/**
+	 * Indicates that the application is running in Firefox (Gecko).
+	 *
+	 * @static
+	 * @member {Boolean} module:utils/env~env#isEdge
+	 */
+	isGecko: isGecko( userAgent )
 };
 
 export default env;
@@ -55,3 +63,13 @@ export function isMac( userAgent ) {
 export function isEdge( userAgent ) {
 	return !!userAgent.match( /edge\/(\d+.?\d*)/ );
 }
+
+/**
+ * Checks if User Agent represented by the string is Firefox (Gecko).
+ *
+ * @param {String} userAgent **Lowercase** `navigator.userAgent` string.
+ * @returns {Boolean} Whether User Agent is Firefox or not.
+ */
+export function isGecko( userAgent ) {
+	return !!userAgent.match( /gecko\/\d+/ );
+}
diff --git a/tests/env.js b/tests/env.js
index 91fb373..2cae15a 100644
--- a/tests/env.js
+++ b/tests/env.js
@@ -3,7 +3,7 @@
  * For licensing, see LICENSE.md.
  */
 
-import env, { isEdge, isMac } from '../src/env';
+import env, { isEdge, isMac, isGecko } from '../src/env';
 
 function toLowerCase( str ) {
 	return str.toLowerCase();
@@ -29,6 +29,12 @@ describe( 'Env', () => {
 		} );
 	} );
 
+	describe( 'isGecko', () => {
+		it( 'is a boolean', () => {
+			expect( env.isGecko ).to.be.a( 'boolean' );
+		} );
+	} );
+
 	describe( 'isMac()', () => {
 		it( 'returns true for macintosh UA strings', () => {
 			expect( isMac( 'macintosh' ) ).to.be.true;
@@ -75,4 +81,26 @@ describe( 'Env', () => {
 			) ) ).to.be.false;
 		} );
 	} );
+
+	describe( 'isGecko()', () => {
+		it( 'returns true for Firefox UA strings', () => {
+			expect( isGecko( 'gecko/42' ) ).to.be.true;
+			expect( isGecko( 'foo gecko/42 bar' ) ).to.be.true;
+
+			expect( isGecko( toLowerCase(
+				'mozilla/5.0 (macintosh; intel mac os x 10.13; rv:62.0) gecko/20100101 firefox/62.0'
+			) ) ).to.be.true;
+		} );
+
+		it( 'returns false for non–Edge UA strings', () => {
+			expect( isGecko( '' ) ).to.be.false;
+			expect( isGecko( 'foo' ) ).to.be.false;
+			expect( isGecko( 'Mozilla' ) ).to.be.false;
+
+			// Chrome
+			expect( isGecko( toLowerCase(
+				'Mozilla/5.0 (Windows NT 6.2; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.90 Safari/537.36'
+			) ) ).to.be.false;
+		} );
+	} );
 } );