I still use Fiddler. I prefer it to browser inspector network tab when I need to get into the weeds. It does so much, and I can write custom proxy tweaks with Javascript (actually JScript.NET, but works just like good old JS), and it works with other software, I use it with NodeJS all the time. It's like the Swiss army knife of HTTP.
Netscape had a JavaScript debugger, IE had a debugger as well. What firebug did was pull the inspector, debugger, console, and everything together in a really nice dev experience. The goal posts were moved far back and the major browsers hurriedly released their “dev tools” to counter FireBug. Chrome being the first after, followed shortly by Safari (who already had an inspector). It would take MS another 6 years to do the same for IE8.
