Thanks. I forgot to mention my tests were done locally (on XAMPP server). File transfers were theref

Thanks. I forgot to mention my tests were done locally (on XAMPP server). File transfers were therefore from one disk to another (or on the same), and take just several seconds or so. So it can't be the router or internet service provider. And >250MB file upload works ok in other browsers, so I am guessing the issue is specific to Firefox. I will see how it behaves in uploads to the real server. But at the moment, I have issue with uploading large files (>50-100MB) on the real server (troubleshooting now) in ALL browsers. Something in the chain: Browser-->Ajax-->Router-->ISP-->PHP settings(php.ini, .htaccess)-->Server is wrong or has limits set.