I can't say for sure. It's possible Youtube made some changes to their website that don't allow Firefox to upload videos anymore. Do you see any error messages? But maybe it could a cache issue. Could you try clearing the Firefox site data of Youtube.com to see if that allows video uploads to work again? Instructions: https://support.mozilla.org/en-US/kb/clear-browsing-history-firefox-ios#w_clear-individual-site-data