Recommend Good Firefox extensions for developers? (I'll show you mine if you show me yours...)

[eluser]John_Betong[/eluser]
 
Html Validator
 
Immediately after your webpage displays an icon appears in the bottom right had corner to indicate HTML warnings, errors, etc. Red for problem, green for Ok.

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) ...

 
Clicking on the icon opens this page:

 

Simple and great for debugging Html errors.