View Source has been available since "The Beginning"; its how we learned html back in the day. As @Aredridel says, (the grandfather of all browsers) Mosaic had it. And since Internet Explorer was based on Mosaic, IE had it View Source from the start too.

Here's a good history lesson on "The Birth of Mosaic": http://www.nsf.gov/news/special_reports/nsf-net/bina.jsp

First tool I ever used that I remember being good was Firebug which I was using around 2006-2007. That was one of my main reasons for using Firefox at that point. Once Google Chrome came along with it's inspector, I switched browsers.

Like said, the "view source" as viewing the html text source code has been around for ever. But the "Inspect Element" OR "visual representation of the DOM Tree" has been established AT LEAST since IE8 with the F12 Developer Tools.

Netscape 3 had so.e rudimentary dev tools, and a WYSIWYG editor that were dev tools, abd a JavaScript console.

View source has been around forever. NCSA Mosaic had it.