| Language Manager | | - | | |
| KillSpinners | | - | | |
| Just Restart! | | - | | |
| Just Disable Stuff | | - | | |
| JavaScript Debugger | | - | | |
| JavaScript Actions Menu | | - | | |
| JavaScript Actions | | - | | |
| JSOff | | - | | |
| JSActions K | | - | | |
| JS Switch | | - | | |
| Is It Compatible? | | - | | |
| Integrated Authentication for Firefox | | - | | |
| InspectorWidget | | - | | |
| InfoLister | | - | | |
| Imitate IE | | - | | |
| Image Block | | - | | |
| IMEStatus | | - | | |
| IME-Colors | | - | | |
| IME and DiMENSiON | | - | | |
| Hide Preferences | | - | | |
| Hachima Go To Hell（はちま禁止） | | - | | |
| HTML5Toggle - Toggle HTML5 support on or off | | - | | |
| Get Mail | | - | | |
| Geolocater | | - | | |
| Free Memory Button | | - | | |
| Free Memory 2.0 | | - | | |
| Free Memory | | - | | |
| FoxyPerformance | | - | | |
| FoxyAbout | | - | | |
| Fox Restart | | - | | |
| Font Smoothing Adjustment | | - | | |
| Font Size | | - | | |
| Font Setter | | - | | |
| Flash OnOff | | - | | |
| Flash Block | | - | | |
| Firesizer | | - | | |
| Firesize | | - | | |
| FireSwitch | | - | | |
| FireFox Tweak | | - | | |
| Fire Resizer | | - | | |
| FindList | | - | | |
| FeatureFix | | - | | |
| FavLoc | | - | | |
| Fat-Free Firefox | | - | | |
| Fasterfox Lite | | - | | |
| FEBE | | - | | |
| Extensor | | - | | |
| Extension Options Menu | | - | | |
| Extension List Dumper 2 | | - | | |
| Extension List Dumper | | - | | |