Firefox is my favourite browser for long time, as it is for many intermediate to advanced users. The thing I like most with Firefox is that, it comes bare and stable, and I can extend it in my taste. That enables me to use it in the most efficient and personalized way. Furthermore, it has a portable version that I can carry everywhere with my own settings using a memory stick.
If you are a web developer you may like this post. It extends firefox with most useful and popular web developer extensions and gives short definitions of them. Eye dropper, link checker, screen capturer and HTML validator are some gripping ones.