Problem: Cannot paste anything in a YouTube comment.
At first everything worked fine. Then URLs could not be pasted, and now nothing can be. Perhaps this is a problem with Pale Moon compatibility, and perhaps this is an issue with my Pale Moon extensions. I didn't look deeply into this.
Solution: Create a top-level comment, reply to it and paste your text there.
If YouTube intentionally created this problem to, for example, block spam comments, and they decide to be more aggressive about this and block pasting in sub-comments, then another solution will be investigated. I recall having a way around this sort of problem when pasting a password into one website was blocked.