Opera used to have its own debugging/inspection tool[1] as well. It was released in 2008, it was open-source[2] (unlike the browser), didn't required plugins like firebug at the time, had handy remote debugging, overall I enjoyed it the most. But it died with the browser. Honestly, I'm surprised it's still up somewhere.

1. https://www.opera.com/dragonfly/

2. https://github.com/operasoftware/dragonfly