Mozilla finally added an environment variable to control this behavior. Unfortunately, configuring this environment variable in a way that applies to the overall graphical system, rather than merely a bash session, is a bit difficult. This used to be done via /etc/launchd.conf, but macOS dropped support for this in v10.10. Fortunately, systemctl offers a .plist file system which can define run programs and define system-wide environment variables at boot, so I published this working .plist file, with instructions for installing and removing it:

https://github.com/mcandre/dotfiles/blob/master/setenv.MOZ_DISABLE_SAFE_MODE_KEY.plist

This is awesome for me, because I like to launch my web browser from anywhere in the GUI with Control+Alt+G via QuickSilver, which of course includes the Alt modifier that Firefox tends to interpret as signaling safe mode.