Chrome browser

XPath Helper

A nice plugin for testing XPATH expressions directly into Chrome. Useful if you want to test how to extract bits and bobs from a page without waiting for a site crawl to be carried out.

Webclipper’:
This neat little plugin will help you extract your SEO related stories more efficiently into Evernote.

Ghostery

Find tracking scripts installed on a web page.

Redirect Path

Check redirects, 404 errors as you visit pages.

Tag Assistant (by Google)

Helps to troubleshoot installation of various Google tags including Google Analytics, Google Tag Manager and more.

Chrome UA Spoofer

Spoofs & Mimics User-Agent strings.

Open SEO Stats

Shows web rank and SEO stats of current web page, quick access to Geo IP Location, Whois, Alexa, backlinks, indexed pages, etc

ExtensionIP Domain Country Flag

Shows country flag and other IP / domain information in the location bar.

See robots

SeeRobots displays the meta robots information of the website you are viewing. This tool is especially useful to see if a noindex tag has been added.

Yslow

Page speed checker plugin.

Canonical

This plugin will show in Blue when a page has a canonical tag. If you click it you will be forwarded to the destination URL to check further.

Data Scraper

Data Scraper extracts data out of a web page and imports it into Microsoft Excel spreadsheets. Data can be scraped via Jquery or Xpath. It can even be used for scraping SERPS (10 results at a time).

MozBar

The MozBar lets you create custom searches, compare link metrics, highlight links and keywords, quickly expose page elements, and access other powerful SEO tools—all in-line with your browser.

Web Developer

Adds a toolbar button with various web developer tools. The most useful are a) Disable JavaScript b) Disable CSS

Image Size Info

This extension allows you to view image width, height and file size via the right click menu. menu.

Firefox

Web Developer

Web developer tools

Live headers

View HTTP headers of a page and while browsing.

SearchStatus
Alexa rank, Compete ranking and SEOmoz Linkscape mozRank anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, Alexa info and other SEO tools.

User Agent Switcher

Adds a menu and a toolbar button to switch the user agent of a browser to a crawler bot, mobile or custom one.