I'll skip the alpha, might try the beta if it proves reliable from many users and has something new to offer...
I use FF with adblock, fasterfox (on settings which don't hammer servers), and mcafee site advisor to warn me of bad websites.
I read that a new extension which is a beta available now is like noscript in that it blocks scripts, but it keeps a file record of bad scripts to automatically blacklist for you. I will install that one the day it is out of beta form.
I really like the streamlined layout you can customize in FF, maximizing screen space. Not sure if you can do that in opera.
Opera unaltered might have been faster than FF but I couldn't really tell, I tried some customizations and messed it up to where pages loaded funny. I'm sure it is more secure and the odd behavior was from me messing it up somehow but FF is just very easy to set up the way you like.
My only knock on FF is the memory leaks. The other day for work and leisure I was on the computer for about 4 hours with tons of myspace pages up, work related sites, and music vids from youtube playing in the background, I had to restart FF because it gradually ate more & more memory. Other than that its been perfect.