| ThumbsDown | | - | | |
| Tamper Data | | - | | |
| TFToolbarX | | - | | |
| TAB TRAFFIC MONITOR | | - | | |
| SwitchHosts | | - | | |
| Swap Proxy | | - | | |
| Sutare_Extension | | - | | |
| Super User Agent | | - | | |
| Strict Transport Security - Declarative Security Detector | | - | | |
| Stealthy | | - | | |
| Status Watch | | - | | |
| Speed Tweaks (SpeedyFox) | | - | | |
| Sothink Web Video Downloader for Firefox | | - | | |
| Socks Switcher | | - | | |
| Socks Proxy | | - | | |
| Smart HTTPS | | - | | |
| SixOrNot | | - | | |
| Sitenable | | - | | |
| SiteDelta Watch | | - | | |
| SiteDelta Highlight | | - | | |
| SingleFile | | - | | |
| SimpleSessionDetection | | - | | |
| Simple mass downloader | | - | | |
| Simple download notifier | | - | | |
| Simple Mail | | - | | |
| ShowIP | | - | | |
| Show Website Verifier | | - | | |
| Show MyIP | | - | | |
| Show Host IP | | - | | |
| Show File Size2 | | - | | |
| Show File Size | | - | | |
| ShiftLeftClickDownloader | | - | | |
| Shelve | | - | | |
| Session Keeper | | - | | |
| Server Spy | | - | | |
| Semantic Checker | | - | | |
| Seerobots | | - | | |
| Secure Or Not | | - | | |
| SaveLink | | - | | |
| Save Selected Tabs to Files | | - | | |
| Save Link in Folder | | - | | |
| Save In… | | - | | |
| Save Image in Folder | | - | | |
| Save Image In Folder (861310) | | - | | |
| Save File to | | - | | |
| SSLPersonas | | - | | |
| SSL Version Control | | - | | |
| SPDY indicator | | - | | |
| S3.Download Statusbar | | - | | |
| Rewrite HTTP Headers (JS) | | - | | |