diff --git a/CHANGES.md b/CHANGES.md
index 4e32b4789e3..f40288575f0 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -3,6 +3,7 @@ CKEditor 4 Changelog
 
 ## CKEditor 4.3.2
 
+* [#11121](http://dev.ckeditor.com/ticket/11121): [Firefox] Fixed: High Contrast mode is enabled when editor is loaded in hidden iframe.
 * [#11350](http://dev.ckeditor.com/ticket/11350): The default value of [`config.contentsCss`](http://docs.ckeditor.com/#!/api/CKEDITOR.config-cfg-contentsCss) is affected by [`CKEDITOR.getUrl`](http://docs.ckeditor.com/#!/api/CKEDITOR-method-getUrl).
 * [#11097](http://dev.ckeditor.com/ticket/11097): Improved the [Autogrow](http://ckeditor.com/addon/autogrow) plugin performance when dealing with very big tables.
 * [#11290](http://dev.ckeditor.com/ticket/11290): Removed redundant code in `sourcedialog` plugin.
diff --git a/core/_bootstrap.js b/core/_bootstrap.js
index be568814a4f..0ca96d10920 100644
--- a/core/_bootstrap.js
+++ b/core/_bootstrap.js
@@ -13,15 +13,20 @@
 		CKEDITOR.env.hc = false;
 	else {
 		// Check whether high contrast is active by creating a colored border.
-		var hcDetect = CKEDITOR.dom.element.createFromHtml( '<div style="width:0px;height:0px;position:absolute;left:-10000px;' +
-			'border: 1px solid;border-color: red blue;"></div>', CKEDITOR.document );
+		var hcDetect = CKEDITOR.dom.element.createFromHtml( '<div style="width:0;height:0;position:absolute;left:-10000px;' +
+			'border:1px solid;border-color:red blue"></div>', CKEDITOR.document );
 
 		hcDetect.appendTo( CKEDITOR.document.getHead() );
 
 		// Update CKEDITOR.env.
 		// Catch exception needed sometimes for FF. (#4230)
 		try {
-			CKEDITOR.env.hc = hcDetect.getComputedStyle( 'border-top-color' ) == hcDetect.getComputedStyle( 'border-right-color' );
+			var top = hcDetect.getComputedStyle( 'border-top-color' ),
+				right = hcDetect.getComputedStyle( 'border-right-color' );
+
+			// We need to check if getComputedStyle returned any value, because on FF
+			// it returnes empty string if CKEditor is loaded in hidden iframe. (#11121)
+			CKEDITOR.env.hc = !!( top && top == right );
 		} catch ( e ) {
 			CKEDITOR.env.hc = false;
 		}