1.Enter Fastboot mode,flash recovery:
D:\BaiduYunDownload\recovery>fastboot flash recovery 6.0.4.7-NonTouch.img
sending ‘recovery‘ (11796 KB)...
OKAY [ 0.489s]
writing ‘recovery‘...
OKAY [ 0.955s]
finished. total time: 1.446s
2.Enter Recover mode -> install zip -> install zip from sideload,Now Nexus waitting for sideload。
3.In PC run：
C:\Program Files (x86)\QTShuaJi\AdbAdapter>adb sideload D:\BaiduYunDownload\cm-
12.1-20150901-SNAPSHOT-YOG4PAO237-hammerhead.zip
3.Go back-> reboot system
Nexus 5 Change FireFox OS to android
原文：http://www.cnblogs.com/xiaoxiaoleo/p/5887102.html