YouTube videos keep pausing and buffering intermittently in Firefox

Since you already cleared the cache and ruled out extensions, I’d test whether it’s actually Firefox or the connection first. Try the same YouTube video in Chrome on the phone. Also switch between Wi-Fi and mobile data. If it only happens in Firefox, clear the YouTube site data, then fully close and reopen Firefox. I’d also make sure Firefox and Android are up to date and temporarily turn off Battery Saver/Data Saver, since either can interfere with video playback. If it still happens, tell us the phone model, Firefox version, and whether YouTube works normally in Chrome. That would narrow it down quite a bit.