A collection of SEO, SEM & Webmaster tools for analysing and optimising websites.
The Web Developer extension adds various web developer tools to the browser.
The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.
Displays a country flag depicting the location of the current website's server and provides a multitude of tools such as site safety checks, whois, translation, similar sites, validation, URL shortening, and more...
View HTTP headers of a page and while browsing.
IE Tab - an extension from Taiwan, features: Embedding Internet Explorer in tabs of Mozilla/Firefox...
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
Seoquake is a Firefox SEO extension aimed primarily at helping web masters who deal with search engine optimization (SEO), social media optimization (SMO) and internet promotion. Seoquake allows to investigate many important SEO parameters.
The extension previously known as GooglePreview. Inserts preview images (thumbnails) and popularity ranks of web sites into the Google, Yahoo, Bing and DuckDuckGo search results pages.
Access important SEO metrics while you surf the web. This toolbar from Moz lets you see metrics like Domain Authority and Page Authority for every page you visit, analyze links as you browse, expose page elements, and access other SEO tools.
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon
For Firefox 23+, you need 0.958 (under review)
Display the Google PageRank, Alexa rank, Compete ranking and SEOmoz Linkscape mozRank anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, canonical links, WHOIS, robots and more.