In Annozilla, you can annotate either the entire document, or a region of it.
To annotate a region, just select that region. To annotate the document, make sure there is no current selection.
Then, in the sidebar, select Annotations -> Create Annotation.
This will open an editor window. Type your annotation, then select Post when you've finished.
(does this automatically insert the annotation onto the list in the left pane?)