When you double-click text in Firefox, what is selected depends on the punctuation characters that are inside or around the text. By default, punctuation characters like dashes are not part of the word. So, you might find that a word-like-this is not fully selected on double-click.
This selection behavior is controlled by the preference layout.word_select.stop_at_punctuation. You can go to about:config and change it if you would prefer that punctuation characters be ignored.