What I like about Firefox is the fact that it's open source and is improved upon quite often....almost daily, at times. It is not the result of committee meetings, office politics and disgruntled Microsoft programmers working under someone else's thumb.
That's what got me to try it but it's the extensions that won me over. Before Firefox I was a big Opera fan but with extensions, I've given Firefox alot of what I liked about Opera and much more.