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Firefox responsive design mode - cannot add custom device

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The tech note at https://developer.mozilla.org/en-US/docs/Tools/Responsive_Design_Mode shows a GIF anim of how to add a custom device and user-agent. There should be a button called "Add Device" under "Edit List" but I don't have this.

I've restarted with add-ons disabled, refreshed FireFox, all without any luck. Is there a setting I'm missing?

The tech note at https://developer.mozilla.org/en-US/docs/Tools/Responsive_Design_Mode shows a GIF anim of how to add a custom device and user-agent. There should be a button called "Add Device" under "Edit List" but I don't have this. I've restarted with add-ons disabled, refreshed FireFox, all without any luck. Is there a setting I'm missing?
Attached screenshots

Modified by redsundesign

Chosen solution

Apologies - I really should have read the tech note more closely.

Feature not available until FireFox 54. Looking forward to that release.

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Apologies - I really should have read the tech note more closely.

Feature not available until FireFox 54. Looking forward to that release.