Ergo, why not have these particular features integrated?
- Mail and Newsgroups: Straight up, having your eMails in your browser without needing to go to Google Mail, AOL, Yahoo or whatnot
- Composer: Allows HTML editing in a WYSIWYG editor
- Address Book: A portion of the above Mail and Newsgroups wicket
- Chatzilla: Built-in IRC system (works better than ... mibbet)
Still, the other three should warrant some attention. As to why they were never considered when Mozilla 'original' had these features, I seriously question. And yes, I worked with the browser when it still used the name Mozilla.
EDIT: Chatzilla may be found by way of google. But the last update in that edition was updated in 2016 and not compliant with current Firefox.