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Open a new Firefox tab or window, and type about:config (If a warning message comes up, just click OK).
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Create a new entry
devtools.responsiveUI.presets
- right click and chose 'new' then 'string' - under enter the preference name typedevtools.responsiveUI.presets
then hit OK. When the enter string box appears simply add the presets. -
Restart Firefox.
Obs: It's devtools.responsiveUI.presets, not devtools.responsive.UI.presets.