Building Mozilla for Symbian OS is in research stage now. The method and process mentioned here is subject to change greatly as the project goes on.
Overview
Our first milestone is to make XULRunner run on a cell phone with S60 3rd Edition platform whose underlying operating system is Symbian OS. To simplify this target, say we are porting Mozilla FireFox onto Symbian OS.
What has been done now
See bug 432430, you will find the patch which will make NSPR built on Symbian OS. But this patch only give you a success build, the result is not tested at all. I believe there are many fixes to do until the NSPR on Symbian OS could work as expected.
How to build
Before build NSPR for Symbian OS, it is assumed you know the build process and tool chain used to build a common Symbian OS project. If you haven't got the idea of running a Symbian OS project building, check this and this, the last part "Resources" will help too.
Then you can grab the patch, apply it under directory mozilla/nsprpub. After patch applied, you can use bldmake bldfiles and abld build gcce urel command to build the project under path mozilla/nsprpub/build/symbian. Also you could import bld.inf projects set file in your Carbide.C++ IDE and then build it.
What's in this patch
What need to be done next
Test NSPR and continue porting other components like NSS, necko, xpcom and etc.