This is Google’s strategy. Haven’t you followed the manifest V3 debacle? They want to end ad blocking once and for all. Their entire business model is to sell ads. They want to turn that ad blocking crooks into sweet new ad revenue. Maybe even subscription revenue.
In what way are they sacrificing their monopoly? There’s no viable alternative to n YouTube.
They also restricted IE6 when it was far more dominant than Firefox is today (and when YouTube was far less dominant), so it’s not completely unheard of.
They can probably find loop holes, like saying they do support many alternative browsers like Edge, Safari, Opera, Vivaldi, Brave, etc. . They just don’t want “insecure” and “outdated” browsers that support terrible stuff like ad blocking, but they can agree to support Firefox if Mozilla takes action to prevent “insecure” extensions like ad blocking.