How to run Firefox in Protected Mode? (i.e. at low integrity level)

Unfortunately there is currently no way of running Firefox in Protected Mode.

If you're not running 64-bit Windows, you can get something somewhat similar using Sandboxie.

You can run Firefox in low integrity mode using the following commands:

icacls "C:\Program Files\Mozilla Firefox\Firefox.exe" /setintegritylevel low
icacls "C:\Program Files\Mozilla Firefox" /setintegritylevel(oi)(ci) low /t

icacls "C:\Users\*username*\AppData\Local\Temp" /setintegritylevel(oi)(ci) low /t
icacls "C:\Users\*username*\AppData\Local\Mozilla" /setintegritylevel(oi)(ci) low /t
icacls "C:\Users\*username*\AppData\Roaming\Mozilla" /setintegritylevel(oi)(ci) low /t
icacls "C:\Users\*username*\Downloads" /setintegritylevel(oi)(ci) low /t

Note that you'll need to run the second batch for each user on your system, customising username, otherwise they'll get a "Firefox is already running" message box.

However this setup does cause the following quirks:

1. Profile manager may not behave correctly.
2. You'll get a security warning every time you start Firefox.
3. Downloads can only be placed in low integrity directories (hence Downloads is marked as low integrity above).
4. Opening downloads directly from Firefox will generally fail.