It updates Ublocks filters. They are updating those as YouTube changes things. Ublock has worked flawlessly on firefox YouTube for me so far. I have not even seen the popup once.
Plus you should add Anti-adblock killer by Reek, a combination of those 2 makes the entire problem go away... for now. I would also whitelist youtube in other addons like disconnect as those can also trigger youtubes immune response.
Read the thread thoroughly, you may have missed a step or 2, or may have misunderstood something. There's a lot of noise in the replies. Filter maintainers can't always give support or keep up with the noise. Being pushy, unfriendly, or replying to the thread without doing every step will not fix the problem for you. If you have technical knowledge, consider contributing that knowledge.
Do not add random filters, scripts, extensions from random users. If you don't understand what you're doing you may add something that may work against you instead of for you, or in worse cases add something malicious.
YouTube seems to update their anti-ab script at least twice a day more or less. So just: Click 🛡️ uBO's icon > the ⚙ Dashboard button > the Filter lists pane > the 🕘 clock icon at the end of the uBlock filters – Quick fixes list > the 🔃 Update now button. Do NOT click purge all caches and update now, that will only add more pressure to the filter hosts.
If there's a mismatch between the filter fixes and YouTube's latest fixes, just wait, watch the video logged out, or use an alternative front-end like piped, invidious, or whatever.