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Html Validator

HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon

For Online HTML 5 validation, you need 0.962
http://users.skynet.be/mgueury/mozilla

Rated 4 out of 5 stars (214)
55,954 users

Random Agent Spoofer No Restart

Rotates complete browser profiles ( from real browsers / devices ) at a user defined time interval. It includes many extra privacy enhancing options

Rated 5 out of 5 stars (113)
52,958 users

JS Print Setup

Client side Javascript printer settings.
This extension implements print setup from CS Javascript, similar of MeadCo's ScriptX ActiveX control for Internet Explorer.

Rated 5 out of 5 stars (22)
52,510 users

ShowIP

Show the IP address of the current page in the status bar. It also allows querying custom information services by IP (right click) and see location and avaiable vouchers for shopping sites. Additionally you can copy the IP address to the clipboard.

Rated 3 out of 5 stars (285)
52,181 users

Proxy Switcher No Restart Featured

Access to the complete Firefox's proxy settings right from a toolbar panel

Rated 5 out of 5 stars (11)
50,403 users

User-Agent Switcher No Restart Featured

Quickly and easily switch between popular user-agent strings.

Rated 4 out of 5 stars (20)
50,092 users

Poster

A developer tool for interacting with web services and other web resources that lets you make HTTP requests, set the entity body, and content type. This allows you to interact with web services and inspect the results...

Rated 4 out of 5 stars (62)
48,175 users

Extended Statusbar No Restart

A Statusbar with speed, percentage, time and loaded size (similar to Opera's one)

Rated 4 out of 5 stars (250)
46,858 users

FireMobileSimulator

this add-on simulates japanese mobile device.
this add-on support DoCoMo, Au(KDDI) and SoftBank device.

Rated 4 out of 5 stars (11)
46,544 users

FirePHP

FirePHP enables you to log to your Firebug Console using a simple PHP method call.

Not using Firebug 1.9+ on Firefox 8+? See http://firephp.org/

Rated 4 out of 5 stars (48)
46,217 users

HTTP/2 and SPDY indicator No Restart

An indicator showing HTTP/2 and SPDY support in the address bar.

Rated 4 out of 5 stars (20)
45,184 users

SEO Toolbar by Moz

Get SEO metrics while you surf! For each page you visit, see metrics like Domain and Page Authority, analyze links, expose page elements, and access other SEO tools. Next version will be for both Firefox and Chrome. Stay tuned!(This is version 2.6.5)

Rated 4 out of 5 stars (43)
40,910 users

Classic Toolbar Buttons No Restart Featured

'Classic Toolbar Buttons' add-on restores old toolbar button ui (known from Fx 4-7s small view, Fx 4-12s large view & Tb 5-14) for current Firefox, Thunderbird and Seamonkey versions on Windows.

Beta builds on versions page!

Rated 5 out of 5 stars (150)
40,077 users

Tiny JavaScript Debugger

TinyJSD is a JavaScript debugger for privileged code running Mozilla products like Firefox, Thunderbird, SeaMonkey. It serves to debug the application as well as extensions written in JavaScript.

Rated 4 out of 5 stars (6)
39,593 users

Abduction! - Webpage Screenshots Screen Capture

1) Capture an page or just a specific region.
2) Adds a right click option to take screen shots of an entire web page or just part of a web page to save as an image.

VIDEO: http://youtu.be/vpnPvNShpWQ

Rated 4 out of 5 stars (522)
38,545 users

Advanced Cookie Manager

A must have tool for Web Developers and Testers to Manage and monitor web cookies.

Features List:
Add
Delete
Modify
Export / Backup
Import / Restore
Monitor cookies

Available in English, French, Spanish, German and Italian languages.

Rated 4 out of 5 stars (35)
37,592 users

about:addons-memory No Restart

Adds an about:addons-memory page that details add-on memory usage

Rated 4 out of 5 stars (117)
36,124 users

Font Finder Featured

FontFinder is created for designers, developers and typographers. It allows a user to analyze the font information of any element on a page, copy any piece(s) of that information to the clipboard, and perform inline replacements to test new layouts.

Rated 4 out of 5 stars (68)
35,363 users

Disable WebRTC No Restart

WebRTC leaks your actual IP addresses from behind your VPN, by default.

Rated 5 out of 5 stars (27)
34,852 users

HackBar

Simple security audit / Penetration test tool.

Rated 4 out of 5 stars (41)
33,252 users