Latest Visual Composer have problems in Firefox and some Google Chrome versions in WordPress 4.9 (WP 4.9 version is unstable and have a lot of bugs). If you already updated to WP 4.9 try this:
1) Use frontend editor in Visual Composer (not backend editor)
2) Use another browser (Safari, latest Google Chrome, Edge)
This issues should be fixed in next Visual Composer plugin update, WordPress update (or browser updates). When plugin update will be released we will include it to our theme update asap.