I love Firefox, and Iron (a more privacy-enhanced version of Chrome), but there will always be a special spot in my heart for Opera. I've been using Opera for about 8 years now and I've written about it before.
As far as I can remember, speed has always Opera's main strength. Not blazingly fast but noticeably fast. It crashes less than other browser. It's far from being the perfect browser but in many ways Opera seems to be on the right track.
I could go on and on about the technical parts, but it's the little things that makes me stick with Opera all these years.
One of it is the fit to width feature. It makes any large web page fit your monitor's width. I prefer to not scroll horizontally, and fit to width solves this issue for me with a single mouse click.
Zooming in Opera is better than in others. A lot of times when the fonts on a web page is small, I open it in Opera and hold down the CTRL key while moving the mouse wheel to adjust the size. Opera resizes the text size and the size of images. (I think the newly released Firefox 3.10 also has an improved zooming feature.)
In Opera, we can go to a text web address (one this is not linked but in text only) by highlighting it, right click and selecting Go to Web Address. I don't think other browsers have this useful feature, which I think they should.
Well, all this pretty much sums up why I'm an Opera user and supporter. Job well done, Team Opera!
Never tried Opera? Tsk, tsk. Don't wait
[ Opera: Celebrating 15 years of browser innovation ]