"why simple dislike-count extension need to collect this much information about my YouTube activity" - it's explained under every review here. Try reading the response, please. I literally list every point, and why it is there. They are trivial, like detecting what video is playing (to show dislikes for that video) and detecting that like or dislike button was clicked.

The extension is actually recommended by Mozilla, so they do not consider it harmful or dangerous.

Ajay, the author of SponsorBlock and DeArrow, has also raised this issue with Mozilla. He explains how the new wording can make an existing extension look like it suddenly started collecting new data, without making clear which websites it can access. You can read his post on Mozilla’s forum here: https://discourse.mozilla.org/t/the-new-data-transmission-permissions-seem-to-be-misleading-to-users/149410

I'll copy-paste it once more

Personally identifiable information - RYD’s randomly generated, anonymous user ID, used to count votes. This isn’t your Google or YouTube account.

Browsing activity - opening the next YouTube video, so the extension can send its video ID and fetch the new dislike count.

Website content - things like the visible like count, which helps calculate dislike estimates.

Website activity - clicking Like or Dislike.