Most of this is covered in How to customize the toolbar, except the initial paragraph that covers the difference between the Menu bar, Navigation Toolbar, and Bookmarks Toolbar. We should move that over to How to customize the toolbar.
Add-ons (extensions, themes and plugins) and Using the Add-ons window
Most is covered in Customizing Firefox with add-ons and Uninstalling add-ons, except "Further functions such as Visit Home Page and About". I'm not sure we even need to have that documented; but if we want to bother, it can be added to Customizing Firefox with add-ons.
- Menu Reference
- Options window - Main panel
- Tabbed Browsing