Earlier quoted context omitted.
Often, just changing the user agent string so that Firefox appears to be Chrome will let those sites work. I.e. they work fine in Firefox, but are restricted by user agent string checks to reject that browser and/or OS. There are several plugins for Firefox that make this easy.
I've used Firefox and Mozilla for what, 20 years? And I've never bumped into a situation like this. Some very clearly shit internal IT systems had pathetic IE requirements, but never one that said sorry go get chrome
Google drive is another one, where some things stop working depending on the user agent I'm presenting.