If you're building Firefox on Windows 8 like I am you might want to tweak the following settings to reduce the OS' background CPU load while you're building Firefox (some of these settings may also be applicable to Windows 7, but I haven't test this):
- Add your src/object directory to Windows Defender's list of locations excluded from real time scans. I realized that the "Antimalware Service Executable" was using up to 40% CPU utilization during builds before I did this. You can add your src/objdir to the exclude list using the UI at: Windows Defender > Settings > Excluded files and locations.
- Remove your src/objdir from Windows' list of locations to be indexed. I actually did the inverse, and removed my home directory (which my src/objdir was inside) from the list of indexable locations and re-added the specific subdirectories in my home dir that I wanted indexed (Documents, etc) without re-adding my src/objdir.