YouTube 3-dots menu disappearing on desktop

On YouTube, on Firefox, when I try and click the 3-dots menu at the bottom-right of a video, the menu pops up properly, but when I move the mouse pointer to try and click one of the options, the menu closes. This is only happening in my main browser window, it doesn't happen when I try the same thing in a private browsing window. I've tried disabling any extensions that might be interfering, I've cleared the cache, restarted the browser, and the entire computer itself, to no avail. Anyone have any idea why this is happening and how to fix it? It doesn't seem to be related to extensions as the site works fine with extensions on, in a private window, but nothing else seems to work either.