| Unload Tab | | - | | |
| Unified Urlbar Experiment | | - | | |
| Unified Sidebar | | - | | |
| UniconifyPlaces | | - | | |
| Undo Tab @ps | | - | | |
| Undo Closed Tabs Button (805202) | | - | | |
| Undo Closed Tabs Button | | - | | |
| Undo Closed Tab | | - | | |
| Undo Close Tab Replacement | | - | | |
| Undo Close Tab Ex | | - | | |
| Undo Close Tab Button | | - | | |
| Undo Close Tab | | - | | |
| UltraThin Scrollbar | | - | | |
| Ultimate Theme creator | | - | | |
| URL in Titlebar | | - | | |
| URL in Tab Tooltip | | - | | |
| URL Suffixer | | - | | |
| URL Suffix | | - | | |
| URL Lister | | - | | |
| URL Keyword Completer | | - | | |
| URL Dot | | - | | |
| URL Bar Cleaner | | - | | |
| URL Alias | | - | | |
| URL (protocol，hostname，path) in title | | - | | |
| UI Fixer | | - | | |
| Tree Tabs | | - | | |
| Tree Style Tab | | - | | |
| ToufuFavicon | | - | | |
| Totally not Panorama | | - | | |
| TotalToolbar | | - | | |
| TotalStatistics | | - | | |
| Tools Menu Button | | - | | |
| Toolbar Position Changer | | - | | |
| Toolbar Icon Pack | | - | | |
| Toolbar Extras | | - | | |
| Toolbar Buttons | | - | | |
| Toolbar Appmenu Button | | - | | |
| TooManyTabs | | - | | |
| Toggle Tabs Timer | | - | | |
| Toggle Personal Toolbar | | - | | |
| Toggle Find Bar | | - | | |
| TodoFox | | - | | |
| Titlebar Tweaker Plus | | - | | |
| Titlebar Text | | - | | |
| TitleURL | | - | | |
| Tip Tab | | - | | |
| Tiny-Menu | | - | | |
| Tiny Menu | | - | | |
| Timeline Sidebar | | - | | |
| Tiled Tab Groups | | - | | |
