Firebug
Firebug, along with web developer toolbar, changed everything for me. I learnt web development on IE6 by editing html in notepad. I switched to Firefox for tabbed browsing and because it was faster. When I discovered firebug I almost couldn't believe it. What the hell had I been doing up to this point? Why doesn't everyone know about this? Later I discovered that Chrome had it built in and still most web devs hadn't heard of firebug.