i want firefox for my nokia x6

Sorry, Firefox is not available for Symbian phones like the Nokia X6. For more information, see Will Firefox work on my mobile device??

symbian phones are smartphones like android or maemo why you don't publish a browser for symbian?

Android allows developers to write code in languages other than Java. For example the core of Firefox (Gecko) is largely written in C and C++. The Android native development kit allows us to reuse large chunks of code that we share with desktop Firefox. To the best of my knowledge Symbian does not provide this functionality.