I have placed my primary navigation in the left sidebar and want that menu to be sticky when user scrolls the page. This works perfectly in Chrome, Opera and even Internet Explorer. Only Firefox seems to have an issue. Interestingly, in Firefox, if the page is loaded and scrolled content “skips” down (which also does not happen in any of the other browsers). Any idea what is causing this?
Thank you for hinting in the right direction. I use the Autoptimize plugin for WordPress and disabling the plugin, or its CSS optimization or (finally) specifically the “inline and defer CSS” option (which inlines “above the fold CSS” while loading the main autoptimized CSS only after page load) fixes the incorrect Firefox behaviour, i.e. sidebar navigation is now sticky and works.