The flag doesn't show up in my v62 FF either. It's odd that they named the flag "general.config.sandbox_enabled", when by default without the flag, sandboxing is supposed to now be enabled. So I guess one has to add it and set it to false, to disable it. But it sounds like in the Beta version which you are using, it has no effect either way... or maybe the CSS that you used isn't affected by it, if the CSS only includes tweaks still supported by the API.
Re: Mozilla/Firefox did it again
It's odd that they named the flag "general.config.sandbox_enabled", when by default without the flag, sandboxing is supposed to now be enabled. So I guess one has to add it and set it to false, to disable it. But it sounds like in the Beta version which you are using, it has no effect either way... or maybe the CSS that you used isn't affected by it, if the CSS only includes tweaks still supported by the API.