The below add-ons are by general experience and not the only ones, if you need something more specific, you will find it by searching the Mozilla add-ons database or Google (there are a lot other great add-ons..).
The Web Developer extension adds various web developer tools to a browser.
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Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page…
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An extension to View and Manage form history entries (view, edit, delete, selective clean-up, export/import). This extension enables you to selectively delete privacy sensitive information without having to delete the entire history. With the help of powerful regular expressions, information can simply and effectively be retrieved and/or deleted.
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Adblock Plus allows you to regain control of the internet and view the web the way you want to. Adblock Plus allows you to customize your filters with the assistance of a variety of useful features, including a context option for images, a block tab for Flash and Java objects, and a list of blockable items to remove scripts and stylesheets.
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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 in the status bar when browsing. The extension can validate the HTML sent by the server or the HTML in the memory (after Ajax execution).
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IE Tab is a very popular Firefox extension that could be used to embed Internet Explorer into Mozilla Firefox. It is also used by web developers to see how websites would display in Microsoft’s Internet Explorer. Just like the classic IE Tab, the IE Tab 2 updated version allows you to use IE to display web pages in a tab within FireFox. This updated version includes support for FireFox 3.6
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FireFTP is a free, secure, cross-platform FTP/SFTP client for Mozilla Firefox which provides easy and intuitive access to FTP/SFTP servers. Along with transferring your files quickly and efficiently, FireFTP also includes more advanced features such as: directory comparison, syncing directories while navigating, SFTP, SSL encryption, search/filtering, integrity checks, remote editing, drag & drop, file hashing, and much more!
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Generate “Lorem Ipsum” dummy text (from http://www.lipsum.com). Minimum text length: 5 words. Context menu option on form inputs (“Insert Dummy Lipsum”). This option fills automatically the selected field.
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Automate Firefox. Record and replay repetitious work. Whatever you do with Firefox, iMacros can automate it. With iMacros, you can quickly and easily fill out web forms, remember passwords, create a webmail notifier, download information from other sites, scrape the Web and more.
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Colors can be tried out on websites by dragging a color area from the picker, inspector, or library to webpage content, or by selecting an element to paint with the color picker. Color values can be viewed in hex, rgb, and hsl CSS formats.
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