Enable spellchecking on Firefox

The spellchecking was disabled for Firefox on non-Mac systems because
Firefox had a bug and inserts characters (like B) into the content when
enter an access key combination (like Shift+Alt+B).

Firefox 65 fixed the problem by changing the default for
dom.keyboardevent.keypress.dispatch_non_printable_keys_only_system_group_in_content
from false to true.

The old behavior can activated in about:config by setting
dom.keyboardevent.keypress.dispatch_non_printable_keys_only_system_group_in_content = false

This change reverts 29f6b48eeb.

Bug: T177509
Change-Id: I23773b2ea2a154269a5b3064cd8d9e9132051c38

resources/ext.CodeMirror.js

@@ -1,7 +1,6 @@
 ( function () {
 	var useCodeMirror, codeMirror, api, originHooksTextarea, cmTextSelection,
 		$textbox1,
-		enableContentEditable = true,
 		// Keep these modules in sync with CodeMirrorHooks.php
 		codeMirrorCoreModules = [
 			'ext.CodeMirror.lib',
@@ -37,13 +36,6 @@
 		}
 	};
 
-	// Disable spellchecking for Firefox users on non-Mac systems (Bug T95104)
-	if ( navigator.userAgent.indexOf( 'Firefox' ) > -1 &&
-		navigator.userAgent.indexOf( 'Mac' ) === -1
-	) {
-		enableContentEditable = false;
-	}
-
 	// jQuery.textSelection overrides for CodeMirror.
 	// See jQuery.textSelection.js for method documentation
 	cmTextSelection = {
@@ -131,8 +123,8 @@
 					Home: 'goLineLeft',
 					End: 'goLineRight'
 				},
-				inputStyle: enableContentEditable ? 'contenteditable' : 'textarea',
-				spellcheck: enableContentEditable,
+				inputStyle: 'contenteditable',
+				spellcheck: true,
 				viewportMargin: Infinity
 			} );
 			$codeMirror = $( codeMirror.getWrapperElement() );