Make e10s/non-e10s Tor Browsers indistinguishable

Just citing gk:

e10s in its current form probably brings some fingerprinting risks with it. e.g. users of accessibility tools won't have e10s enabled on Windows and macOS at least. Windows XP users with D3D9 support neither. mcs and brade found that showModalDialog() is not available when e10s is enabled etc.

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