Breaks Firefox if you try to open Responsive Design Mode

[rfsbsb]

When you try to open Responsive Design Mode in a private tab, Firefox stops responding.

Tested on:
Private_Tab: 0.2.2
Firefox: 55.0 (64-bit) with e10s enabled
Operating System: macOS Sierra 10.12.6 (16G29)

Might be worth noting that the developer tools works OK, only Responsive Design Mode doesn't work.

[Infocatcher]

To not lose similar bug report: #255