Firebug
As a Firefox extension Firebug is useful to view, edit and inspect HTML, CSS and JavaScript on any web page in real-time. The extension allows you to filter and search for specific errors, download it from GetFirebug.com.

HTML Validator
HTML Validator is a Firefox Mozilla extension that will highlight the number of errors on a HTML page when you are browsing websites.

BrowserShots
BrowserShots is a free, open-source service where you can submit your web URL and see how your website will be presented in a number of browsers.

Xenu’s Link Sleuth
This free extension scours websites for broken links and when these links are fixed it will make it easier for search engines to crawl your website. Among others Xenu’s Link Sleuth will indicate which images don’t have alternative text, show you the sizes of all the images on your website and identify duplicate titles.