How to comment on changed lines in PR?

I’m trying to comment on specific changes in a PR as documented here: Reviewing proposed changes in a pull request - GitHub Docs

The blue plus icon for adding these comments described in step #4 never appears. It seems unlikely to me that it’s a permissions problem since I do have permission to merge the PR as well as make regular, non-line-specific comments. Is there something extra I have to do to enable this functionality that’s not listed in the documentation (and/or that I’m overlooking in it)?

If relevant, using latest Firefox (91.0.2) on Windows.