Unfortunately, the Firebug add-on was discontinued in 2017 and is no longer available for download. More information is available on [https://hacks.mozilla.org/2017/10/saying-goodbye-to-firebug/ the Mozilla blog]. Fortunately, the regular web development tools on the latest version of Firefox encompass most of the same features that Firebug provided previously. If you are a developer, you may also want to look at downloading the [https://www.mozilla.org/firefox/developer/ Firefox Developer Edition]. This is a version of Firefox designed specifically with developers in mind and it can be run alongside your standard version of Firefox.