[Nexus 5 Android Development] aaron.thomas.webster:
Is anyone else having this problem?

What version of TWRP are you using?

Worked fine for me under TWRP here is my log:

Formatting cache using make_ext4fs function.
Creating filesystem with parameters:
Size: 734003200
Block size: 4096
Blocks per group: 32768
Inodes per group: 7472
Inode size: 256
Journal blocks: 2800
Label:
Blocks: 179200
Block groups: 6
Reserved block group size: 47
Created filesystem with 11/44832 inodes and 5813/179200 blocks
warning: wipe_block_device: Wipe via secure discard failed, used discard instead

I:Recreating /cache/recovery folder.
I:Copying file /etc/recovery.fstab to /cache/recovery/recovery.fstab
I:Version number saved to '/cache/recovery/.version'
Updating partition details...
I:Data backup size is 0MB, free: 7099MB.
I:Unable to mount '/usb-otg'
I:Actual block device: '', current file system: 'vfat'
I:Set page: 'action_complete'
I:Set page: 'clear_vars'
I:Set page: 'wipe'
I:Set page: 'main'
I:Set page: 'clear_vars'
I:Set page: 'main2'
I:Set page: 'install'
I:Set page: 'flash_confirm'
I:Set page: 'flash_zip'
I:Set page: 'flash_zip'
Installing '/sdcard/Download/FireFoxOS-Sep-12-2014.zip'...
Checking for MD5 file...
Skipping MD5 check: no MD5 file found
I:Zip does not contain SELinux file_contexts file in its root.
I:Legacy property environment initialized.
This may take some minutes...
Creating filesystem with parameters:
Size: 1073741824
Block size: 4096
Blocks per group: 32768
Inodes per group: 8192
Inode size: 256
Journal blocks: 4096
Label:
Blocks: 262144
Block groups: 8
Reserved block group size: 63
Created filesystem with 11/65536 inodes and 8536/262144 blocks
Mounting /data
Already mounted.
Mounting /system
Extracting files to /system
minzip: Extracted file "/system/app/SprintHiddenMenu.apk"
minzip: Extracted file "/system/app/UpdateSetting.apk"
minzip: Extracted file "/system/app/qcrilmsgtunnel.apk"
minzip: Extracted file "/system/b2g/Throbber-small.gif"
minzip: Extracted file "/system/b2g/application.ini"
minzip: Extracted file "/system/b2g/b2g"
minzip: Extracted file "/system/b2g/chrome.manifest"
minzip: Extracted file "/system/b2g/components/components.manifest"
minzip: Extracted file "/system/b2g/crashreporter.ini"
minzip: Extracted file "/system/b2g/defaults/pref/user.js"
minzip: Extracted file "/system/b2g/defaults/settings.json"
minzip: Extracted file "/system/b2g/dependentlibs.list"
minzip: Extracted file "/system/b2g/dictionaries/en-US.aff"
minzip: Extracted file "/system/b2g/dictionaries/en-US.dic"
minzip: Extracted file "/system/b2g/libfreebl3.so"
minzip: Extracted file "/system/b2g/libmozglue.so"
minzip: Extracted file "/system/b2g/libmozsandbox.so"
minzip: Extracted file "/system/b2g/libnss3.so"
minzip: Extracted file "/system/b2g/libnssckbi.so"
minzip: Extracted file "/system/b2g/libreplace_jemalloc.so"
minzip: Extracted file "/system/b2g/libsoftokn3.so"
minzip: Extracted file "/system/b2g/libxul.so"
minzip: Extracted file "/system/b2g/omni.ja"
minzip: Extracted file "/system/b2g/platform.ini"
minzip: Extracted file "/system/b2g/plugin-container"
minzip: Extracted file "/system/b2g/precomplete"
minzip: Extracted file "/system/b2g/removed-files"
minzip: Extracted file "/system/b2g/run-mozilla.sh"
minzip: Extracted file "/system/b2g/ua-update.json"
minzip: Extracted file "/system/bin/adb"

....
....
....

minzip: Extracted file "/data/local/webapps/video.gaiamobile.org/manifest.webapp"
minzip: Extracted file "/data/local/webapps/wallpaper.gaiamobile.org/application.zip"
minzip: Extracted file "/data/local/webapps/wallpaper.gaiamobile.org/manifest.webapp"
minzip: Extracted file "/data/local/webapps/wappush.gaiamobile.org/application.zip"
minzip: Extracted file "/data/local/webapps/wappush.gaiamobile.org/manifest.webapp"
minzip: Extracted file "/data/local/webapps/webapps.json"
minzip: Extracted file "/data/nativebenchmark/binderAddInts"
minzip: Extracted file "/data/nativetest/InputDispatcher_test/InputDispatcher_test"
minzip: Extracted file "/data/nativetest/InputReader_test/InputReader_test"
Creating symlinks
Setting file permissions
Unmounting ...
Installation complete
script result was [/system]
I:Legacy property environment disabled.
Updating partition details...
I:mount '/data/media/0' '/sdcard' process ended with RC=0
I:Data backup size is 33MB, free: 7064MB.
I:Unable to mount '/usb-otg'
I:Actual block device: '', current file system: 'vfat'
I:mount '/data/media/0' '/sdcard' process ended with RC=0
I:Set page: 'flash_done'
I:Set page: 'clear_vars'
I:Set page: 'install'
I:Set page: 'main'
I:Set page: 'clear_vars'
I:Set page: 'main2'
I:Set page: 'advanced'
I:Set page: 'confirm_action'
I:Set page: 'action_page'
I:Copying file /tmp/recovery.log to /data/media/0/recovery.log