This my take on the topic started at http://tinyurl.com/3fhw7hb from personal experience.

My reasons are below:

1. Firebug – the defacto standard for web development troubleshooting, along with plugins like Google Page Speed, FirePHP, etc

2. Web Developer- another great extension, allows use of lines, clearing cache, disabling Javascript and cookies etc.

3. Other plugins –

4. Firefox 4 is almost at par with Chrome on speed and memory usage, so there is not much difference

5. Easier installation – I can control where I install Firefox not chrome, and I can also have different versions of Firefox, older versions as well as betas and bleeding edge nightlies

6. Profiles – one for development, a clean one for testing, a loaded one for day to day work, almost like virtualization

7. Brief – RSS feed reading extension, the best that there is

8. Configuration – almost the same as Chrome so no advantage here

9. Better memory usage in Firefox 4 which is almost at par with Chrome

10. Its been around longer so I have more experience with it