On Youtube, on a clean, up to date version of Firefox 64-bit, Youtube will not stay signed in, either on its on board hotbar on the left, the main center console in and outside of individual video pages, or both. Attempting to sign in via the normal means usually sends the user back to the main page as if they had not signed in at all (and if the account is signed in for a given attempt, it will only remain so until the page changes, for example when a search is conducted or a video loaded), and the sign in button on the hotbar is completely non-functional. Additionally, Youtube's other functions such as commenting, saving videos, accessing video history for an account, accessing or creating playlists, etc, do not work at all on Firefox 64 in this instance. This remains the case even when all of the troubleshooting options, such as changing assorted cookies settings, cleaning the cookies that are stored, changing how cookies are stored and what sites have permissions to store or save cookies, changing which are deleted regularly and from which sites or how which are to be deleted is determined, running a clean install of the browser, running the browser as an admin, turning off all ad-ons, running the browser in safe or secret modes (and also these as an admin and not, each), adding Youtube as an exception for tracking protection, and so far as completely uninstalling and reinstalling the entire browser do not fix or change the problem. The problem is not persistent on Chrome or Edge, only on Firefox, and changing which Firefox account is attempting to sign in, or which other accounts (Google, Youtube, etc) are signed in also has no change. Another post with the same problem has determined the source to be Kaspersky Antivirus, however the only posted solution in that thread is to turn off the antivirus, which considering that doing so leaves one's computer unprotected is not actually a solution - I'm maintaining this thread in the search for an actual solution. PS - While I will be contacting Kaspersky as well to see if we can come about a solution on their end, the fact that no other browser that has been tried replicates this problem, even in the presence of an active Kaspersky agent, indicates that the problem is not with ''their'' software, but rather with Mozilla specifically.