Sounds like an account corruption problem with one of your YouTube accounts. Repeat test on Chrome with same YouTube account you used on Firefox. Otherwise not a valid test.
Have you checked your computer for virus/malware lately?
Can you check Youtube-Firefox account on a friend's computer or a work computer or a University library computer? If the problem reoccurrs there, good possibility your YouTube account is bad. Retest,with your 2nd YouTube account or brand new YouTube account. Post back result.