Plug-in (computing), a piece of software which enhances another software application and usually cannot be run independently
+ Add-on (Mozilla), a piece of software that enhances and customizes Mozilla-based applications
1) Web Developer
The Firefox Web Developer add-ons is one of the most popular add-ons for web designers and developers. This add-ons contains a variety off essential tools that allow you to troubleshoot problems easily.
FireFTP is a free, secure, browser based FTP client that allows you to easily connect to your ftp via Firefox instead of opening ftp software seperately.
FireShot is an add-on which allows you to take screenshots of your web pages with a lot of functionality like showing arrows, writing text etc.
ColorZilla is an add-on which gives you the easist way to pick the color from your web page without opening any color picker.
6) IE tab
IE tab is very useful for designer and developer both. Using this add-on , you can see the look of IE at a single click in Mozilla Firefox itself.
AutoPager will automatically shows the other pages in bottom . This helps you when you are searching in google and result shows so many pages, you dont need to click on 2nd page for look at results. It will automatically shows other pages at bottom.
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. Just as the name of this add-on implies, HTML Validator is all about making sure that your code is up to par. You can also check pages that you are visiting, and thanks to a little indicator in the corner of your browser, you?ll quickly see if the page is in compliance or has errors. Once you click on the indicator you will see a full version of the code that will identify what the problems are.
9) Fire PHP
FirePHP enables you to log to your Firebug Console using a simple PHP method call. FirePHP is ideally suited for AJAX development where clean JSON or XML responses are required. All data is sent via a set of X-FirePHP-Data response headers. This means that the debugging data will not interfere with the content on your page.
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It will capture what you can see in the window, the entire page, just a selection, a particular frame… basically it saves webpages as images – either to a file, or to the clipboard. It captures Flash too!