There are several ways to add a comment:

The comments are displayed in the Bug Details area. The Comments Toolbar provides additional functionality for working with comments.

The comments which are saved locally have the <new comment> tag instead of the comment author label and can be modified or removed. To do that, right-click the comment and select Edit Comment or Delete Comment.

Once a comment is uploaded to Bugzilla the <new comment> tag is changed to the comment author label and the comment cannot be modified anymore. Instead, the Reply to Comment option becomes available. Right-click the comment to select this option.

The Comments Toolbar provides the following functions: