Viewer Fix slider control drag Firefox

Viewer/src/viewer/defaultViewer.ts

@@ -60,7 +60,8 @@ export class DefaultViewer extends AbstractViewer {
             }, "pointerdown", "#help-button");
 
             this.templateManager.eventManager.registerCallback("navBar", (event: EventCallback) => {
-                let element = <HTMLInputElement>event.event.srcElement;
+                const evt = event.event;
+                const element = <HTMLInputElement>(evt.target);
                 if (!this._currentAnimation) return;
                 const gotoFrame = +element.value / 100 * this._currentAnimation.frames;
                 if (isNaN(gotoFrame)) return;