I've been a FireFox user since version 0.8 or something. What drove me to FireFox was add-ins. The huge amount of tools I could add was calling me, and the tabbed interface was for me a great advance.
It wasn't speed. Back then, browsers were fast enough, maybe because web pages were lean enough. I don't know if it's really page sizes that is now killing me, but web surfing is painful, both in FireFox and Internet Explorer 7. I used both equally as my default browser on Vista since February, and they really feel sluggish. The worse part is opening a new instance. I don't keep my browser open. Why would I? Tabs are fun, but if you're done with all pages, why keep the browser open? It's only one more useless icon to go through when you switch between applications.
In the past 4 years, I've been trying Opera from time to time, without committing to setting it as my default browser. Every time, it felt much faster, much better for browsing. But the same problem kept killing its chances: where are my add-ons? So it usually got uninstalled after a few days. Thanks Opera for the great browser implementation, but no thanks.
Today, mark the date, September 25th 2007, I'm giving Opera a real chance. It will be my default browser for at least a month. At the end of October, I'll share with you my thoughts about it. I've added the del.icio.us buttons, which don't work like I would them to work, and the developer buttons, which gives me only a percent of what the FireFox Web Developer toolbar had. I'll search for better tools, but will live with those in the meantime. But this time, speed has become more important than tools.