From Forum Nokia Wiki

As the Firebug is working only with Firefox browsers, you can not use the full firebug with WRT Widgets, but as there is a lite version of the Firebug that can be run in other browsers including S60 Browsers, you can utilize the Firebug-lite in the WRT Widgets as well.

Firebug lite home pages can be found form: getfirebug.com/lite.html

To use it in a WRT, you should download source codes for the Firebug-lite from getfirebug.com and store them in your WRT Widget folders.

Then just include them in your html file, for example if the source files were stored in firebug folder, you could include required files by adding following lines into the head section:

<link href="firebug/firebug-lite.css" rel="stylesheet" type="text/css">
 <script language="javascript" type="text/javascript" src="firebug/firebug-lite.js"></script>

Then it appears that the initialization is not handled automatically for some reason, so to get the console showing the initialization function needs to be called. This is handled by using following line of code:

firebug.init()

Full example for including Firebug-lite with WRT Widgets can be found from: Firebug_lite.zip