Firebug Download For Firefox 12 ((new)) • Plus

Firefox 12 was released in early 2012. Because modern Firebug versions (like 2.0.x) require much newer browser engines (Firefox 30+ or 53+), you need a version of Firebug that was contemporary to that era.

Search for the "Add-ons" folder in the legacy Mozilla FTP servers. firebug download for firefox 12

I notice you're asking about for Firefox 12 — that's a very old version (Firefox 12 was released in April 2012). Firebug has since been discontinued and is no longer maintained. Firefox 12 was released in early 2012

To get Firebug working on Firefox 12, you need to navigate the archives of the now-discontinued extension. Finding the Right Version firebug download for firefox 12