I know this is one site that seems to have issue with Firefox and eBay is another for some reasons could be due to programming of the Browser some visual ques don't show up on Firefox but will show up on Edge or Chrome. And this could be due to site programming that works better if you use Edge or Chrome. So unless it comes up with Error reporting this is the only way for Firefox devs will get the report and they can look into the issues and what to update to work with those sites. But makes sure to follow the Firefox trouble shooting guides to insure that Addons/Extensions aren't causing the load issue as well. https://support.mozilla.org/en-US/kb/troubleshoot-and-diagnose-firefox-problems