| Validation within Firefox | |
| https://addons.mozilla.org/en-US/firefox/addon/2520 | Offline Page validator 0.3 |
| https://addons.mozilla.org/en-US/firefox/addon/2289 | CSS validator 0.3.2 |
| https://addons.mozilla.org/en-US/firefox/addon/2318 | Total Validator 4.3 |
| https://addons.mozilla.org/en-US/firefox/addon/249 | Html Validator 0.8.4.0 |
| http://addons.mozilla.org/en-US/firefox/addon/2294 | RSS validator 0.3.2 |
| https://addons.mozilla.org/en-US/firefox/addon/1754 | ocPortal validator 1.0.2 |
| https://addons.mozilla.org/en-US/firefox/addon/3939 | Relaxed the HTML Validator 0.9.2 |
| https://addons.mozilla.org/en-US/firefox/addon/2250 | Page validator 0.3.2 |
| https://addons.mozilla.org/en-US/firefox/addon/1815 | Console2 0.3.8 |
| Also useful: | |
| https://www.squarefree.com/bookmarklets/validation.html | Validation Bookmarklets - Jesse Ruderman |
| https://www.squarefree.com/bookmarklets/webdevel.html | Web Development Bookmarklets - Jesse Ruderman |
| http://web-sniffer.net/ | View HTTP Request and Response Header |