To add the Clear Fields buttons to your toolbar, right click on it and select Customize... then drag the buttons to the toolbar. If you want this done automatically, open the Tools menu -> Clear Fields, and select Add Main Toolbar Buttons. The in-field clear buttons are enabled by default since version 4.0.0. You may disable them in the Tools menu. If you can't find the Tools menu, open the Main Menu, click Preferences and select the Menu Bar.

For suggestions or bug reports, please contact me at alex.alexander@gmail.com
For the latest news regarding this extension, visit my blog.

Changelog:

4.1.0
Support for new findbar in Firefox 25+.
4.0.3
Added new, non-translated entries to non-english language files. This should fix the issues some users were having.
4.0.2
Replaced install/update popup dialog with a web page that opens in a new tab
4.0.1
Bug fix: in-field clear button toggle works now
4.0.0
First version with Firefox 4 support New in-field clear buttons