Firebug Lite for Opera 15+
Modify Header Value
Add, modify or remove a header for any request on desired domains.
Responsive Web Design Tester
Responsive Web Design Tester is a quick and easy way to test your responsive website.
Click on any HTML element to get the font info in a popup
Use Redux DevTools in the DevTools
PDF API HTML5
Live HTTP Headers
Monitor all HTTP/HTTPs traffic from your browser.
Edit the Page
Toggles the 'Design Mode' so that you can edit the contents of the page.
WordPress Theme and Plugins Detector
This tool detects theme and plugins used on WordPress sites and displays information about them.
Right Click Validator
Right click on any webpage to validate it using the W3 NUValidator service.
Website Color Changer
Extends the Developer Tools, adding a sidebar for colors manipulation
This extension allows you to add your own custom website specific CSS style sheets.
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!
Edit with Adobe Photoshop
Adds a context menu item to send image links directly to Adobe Photoshop photo editor
Browse JSON like you do it in Finder.
Get Thumbnails or Image to video YouTube
Make thumbnail previews or images from YouTube videos
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, ...
This will take the URL of the current tab and send it to PageSpeed Insights for a web performance analysis.
View all HTTP Requests and Responses sent between the Web browser and the Web server.
Quickly view the HTTP headers of the current tab.
Picture in Play
Indicates whether the `picture` element is in use on a page.
Remove cookie(s) for the current domain
Remove cookie(s) for the current site with just one click
ColorHexa - Color Information and Conversion
Color tool converting and providing information about any color.
Web Developer Checklist
Analyses any web page for violations of best practices
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
Shows the response times and uptime of a desired server. Useful to see how performs internet or a wireless access point.
Website Analysis & SEO Tool
Website SEO audit tool that provides website analysis in SEO, performance, validity, security, accessibility and other criteria.
Google Analytics Debugger
A browser extension for debugging Google Analytics tracking code