Here is my collection of extensions which are very helpfull for my job of SEO consultant. Verified by experiences.
The Web Developer extension adds various web developer tools to the browser.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
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.
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.
InFormEnter adds a small, clickable icon next to every input field in a web form, from where you can select the item to be inserted - no typing required.
Checks for compliance with the contrast levels, brightness and shine in the color combination of foreground and background of textual content based on the requirements of WCAG 1 and WCAG 2.
SEO Global For Google Search™ (Formerly Google Global) is an unobtrusive Firefox extension that allows you to see what the Google search results that you are viewing look like from different geographical locations.
KGen (Keyword Generator) allows you to see what keywords are strong on visited web pages for search engines.
SEO Profesional Toolbar offers many SEO-related tools in one place. Display PageRank, S-Rank and the number of backlinks for visited pages. Stores the history of ranks for selected sites. Highlight words, links, view number of words and more.
Checks to see if Google Analytics is installed on any given page.
LinkExtend provides meta-site-ratings for computer safety, child safety, company ethics, and popularity. Safety results come from eight online services giving you a safer browsing experience. Site links, titles, files and reviews are also included.
Adds advanced search capabilities to the Firefox address bar. Supercharge the "Awesome Bar" with instant search results that appear as you type!