There are lot's of reason why Firefox Mobile isn't available for your specific device I'm sure. Even some Android devices can't run it.

diplomatic answer...want some concrete reason....??

The device runs Symbian. Which is a mobile OS that i guess it might be possible to create a Firefox Mobile edition for. There are a lot of things to take in to consideration such as the hardware and the api's that are available in the OS. Perhaps someone from the Development team could fill you in on this.

Symbian is a Java based operating system. Firefox is programmed in C++. Android has a way for C++ programs to run. Symbian does not to the best of my knowledge. In addition Nokia seems to have chosen a different direction for their future smartphones, Windows 7 mobile.

Symbian is not Java based, see http://en.wikipedia.org/wiki/Symbian. It's written in C++, supports Phyton and Java of course and has a long history as an operating system for mobile devices. It is not as 'hot' in that sense as Android or iOS, nor did Nokia succeed to sell the advantages of this system - thus it is about to die. A pity, being a Psion and Nokia user from day zero, I know the system is very well updatable and by the way much safer than all the others...