For Firefox you need 700 MB for sources and 1.2GB for the build. This is on Fedora-9.
A full rebuild of the browser takes 11 minutes on 4-core 2.6 GHz Xenon or about 80 minutes on 1.1 GHz single-core laptop. But after that minor patching typically leads to 3-5 minutes of wait-time even on the laptop or some seconds on that server using official build instructions. With some knowledge of the build system and depending on the subsystem, one may cut that incremental build time by factor of 10.