Toolbar Buttons is my most popular add-on. It is as far as I can see the biggest button pack for Firefox. It boasts over 100 buttons for Firefox alone. It also includes a number of buttons for Thunderbird and Sunbird, and though it has less buttons for them as it does for Firefox, it easily surpasses any other add-on available.
There is also an online Toolbar Button Maker for making customized versions of this extension with just the buttons you want in it. There is also a version for making Link Buttons to any page on the web.
Text Format Toolbar
The Text Formatting Toolbar is a powerful toolbar for Firefox that helps format text on forums, blogs and wikis that have become so common on the Internet in recent times. It comes with 19 buttons by default and the ability to create your own. It also comes with a composer for writing and saving posts or templates in.
Pastel SVG icons
I have also produced (and am still adding to) an icon set I created called Pastel SVG. It is intended for use in all my extensions and maybe a theme or two. I have released it in the hope that others will also find it useful.
MinimizeToTray Plus is an extension that can minimize an application to the tray. It can also permanently show the icon, and some tasks can be preformed though the icons menu. It also has the ability to make the application load at startup in a minimized form.