Firebug Lite for Opera 15+
Extension source viewer
View source code of extensions in the Chrome Web Store, Firefox or Opera addon gallery. Has button to install from Chrome Web Store.
PDF API HTML5
Right Click Validator
Right click on any webpage to validate it using the W3 NUValidator service.
Custom Style Script
Edit the Page
Toggles the 'Design Mode' so that you can edit the contents of the page.
Disable cookies for current domain.
Browse JSON like you do it in Finder.
GitHub code tree on steroids
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!
Web Developer Checklist
Analyses any web page for violations of best practices
Click on any HTML element to get the font info in a popup
This extension allows you to add your own custom website specific CSS style sheets.
Use Redux DevTools in the DevTools
Flag & WHOIS SEO Domain Full Info
Return the domain's IP address , information about security, localization of server, ping, etc.
View all HTTP Requests and Responses sent between the Web browser and the Web server.
SEO Analysis with Seoptimer
Just one click to SEO Analysis with Seoptimer
Google Analytics Debugger
A browser extension for debugging Google Analytics tracking code
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, ...
Get Thumbnails or Image to video YouTube
Make thumbnail previews or images from YouTube videos
Remove cookie(s) for the current domain
Remove cookie(s) for the current site with just one click
WordPress Theme and Plugins Detector
This tool detects theme and plugins used on WordPress sites and displays information about them.
A webpage form filler that populates context specific random test data into forms elements.
Open in GIMP photo editor
Adds a context menu item to send images directly to GIMP photo editor
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
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.
Picture in Play
Indicates whether the `picture` element is in use on a page.
This extension predicts rating changes for Codeforces. It shows approximate deltas during and after the contest.
Edit with Adobe Photoshop
Adds a context menu item to send image links directly to Adobe Photoshop photo editor