The post mentions both Edge & Firefox, I would be inclined to say it is most likely a result of a plugin/browser incompatibility, particularly the theming ones that are installed.
But on re-enabling/re-configuring the plugin 'broke' the page again, but if I 'Cleanup Plugin Data', on the Plugin Manager page, this also fixed the issue, so this appears to be a workable fix for the issue, but I had to re-configure the required settings.