You can get Firebug from the Mozilla addons site at https://addons.mozilla.org/en-US/firefox/addon/firebug/ this addon is only for desktop Firefox. If you want to debug websites running on Firefox for Android you will need to use the default Firefox web developer tools and follow the directions at https://developer.mozilla.org/en-US/docs/Tools/Remote_Debugging we suggest using the Aurora builds of each https://www.mozilla.org/en-US/firefox/channel/#aurora