I have been using Firefox on a daily basis since 2005-2006.

When Chrome leaped far out ahead in terms of speed, responsiveness, Firefox nearly lost me as a user (as it was quite slow at the time). Since Firefox largely caught up all has been well. I'd give Firefox an 8.5 out of 10 in terms of how happy I am as a user. They would have to do something atrocious to lose me at this point. I also prefer their engine over Chrome in terms of visual rendering.

The sole meaningful complaint I have about Firefox is its memory usage, which still seems high. It wants 1-2gb of ram no matter what I do.