Firebug Lite for Opera 15+
GitHub code tree on steroids
Click on any HTML element to get the font info in a popup
Browserling - Cross-browser testing
Cross-browser test websites in the most popular browsers such as Internet Explorer, Edge, Chrome, Firefox, Safari and Opera. Try for free!
Modify Header Value
Add, modify or remove a header for any request on desired domains.
PDF API HTML5
Live HTTP Headers
Monitor all HTTP/HTTPs traffic from your browser.
Use Redux DevTools in the DevTools
Corser - Authorize CORS requests
Cross-browser extension for authorizing cross-origin resource sharing (CORS) requests. This extension can be installed on browsers supporting the WebExtensions API (https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions) It intercepts all CORS requests and responses headers, and records/adds/changes their values so as to authorized unauthorized CORS requests. Use this extension only for testing purposes
Right Click Validator
Right click on any webpage to validate it using the W3 NUValidator service.
Get Thumbnails or Image to video YouTube
Make thumbnail previews or images from YouTube videos
Browse JSON like you do it in Finder.
Edit the Page
Toggles the 'Design Mode' so that you can edit the contents of the page.
This extension predicts rating changes for Codeforces. It shows approximate deltas during and after the contest.
Live editor for CSS and LESS - Magic CSS
Live preview of CSS/LESS code with an in-page editor. Autocomplete, convert LESS to CSS, beautify, minify, CSS reloader, linter, ...
View all HTTP Requests and Responses sent between the Web browser and the Web server.
Picture in Play
Indicates whether the `picture` element is in use on a page.
WordPress Theme and Plugins Detector
This tool detects theme and plugins used on WordPress sites and displays information about them.
Website Color Changer
Extends the Developer Tools, adding a sidebar for colors manipulation
Remove cookie(s) for the current domain
Remove cookie(s) for the current site with just one click
SEO Analysis with Seoptimer
Just one click to SEO Analysis with Seoptimer
Google Analytics Debugger
A browser extension for debugging Google Analytics tracking code
This extension allows you to add your own custom website specific CSS style sheets.
This will take the URL of the current tab and send it to PageSpeed Insights for a web performance analysis.
Outlines nofollow links, detects nofollow and noindex meta tags on webpages. Features website filtering and custom CSS outline styles
Quickly view the HTTP headers of the current tab.
Website Analysis & SEO Tool
Website SEO audit tool that provides website analysis in SEO, performance, validity, security, accessibility and other criteria.
Flag & WHOIS SEO Domain Full Info
Return the domain's IP address , information about security, localization of server, ping, etc.
Web Developer Checklist
Analyses any web page for violations of best practices