With so many plugins in use now a days, it is impossible to actually see what the best ones are and why we should use them.
Well I stick to a couple of them that I use (on Firefox) when I develop websites. Here is the list.
saves webpages as images...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. http://www.screengrab.org/
Draw out a ruler to get the pixel width and height of any elements on a webpage.
FireFTP is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers. http://fireftp.mozdev.org/
FirePHP enables you to log to your Firebug Console using a simple PHP method call. All data is sent via response headers and will not interfere with the content on your page. FirePHP is ideally suited for AJAX development where clean JSON and XML responses are required. http://www.firephp.org/