First of all compliments to the creator, it's a great tool.
I recently copied the settings from chrome to firefox and div.container doesn't show anything in data preview. IS there anything different in scrape for firefox as compared to chrome?
I don't think it is WS issue, but rather the website could be rendering pages differently for different browsers, i.e. it has detected you are using Firefox and is showing you different version of the page.
You can confirm if your selectors still work by click on Element preview.
preview doesn't work