Currently I do not think there is a way to store screen sizes in the Response Design View. However you should be able to add and remove a preset value. If you have more than the minimum, you can try a different profile with different presets?

Profile Manager - Create, remove or switch Firefox profiles

There is a way to store custom settings for responsive design.

You can create custom settings in the responsive design mode window. You can hold down the Shift key and drag the borders via the (right border, bottom border, bottom right corner) resize icons to get specific dimensions and give the current custom setting a name to add them as a preset. This will add the preset to the devtools.responsiveUI.presets pref that you can inspect on the about:config page.

Those settings are stored in JSON format.

[{"key":"360x640","width":360,"height":640},{"key":"768x1024","width":768,"height":1024},{"key":"800x600","name":"8x6","width":800,"height":600},{"key":"800x1280","width":800,"height":1280},{"key":"980x1280","width":980,"height":1280},{"key":"1280x600","width":1280,"height":600},{"key":"1920x900","width":1920,"height":900}]

See this thread for some JavaScript code to add extra sizes:

• [/questions/957590]

You can paste the existing JSON value of the devtools.responsiveUI.presets pref in the prompt and click OK. Extra values can be added by typing its dimensions (<width>x<height> <name>). You can paste the resulting JSON value back to the devtools.responsiveUI.presets pref or save it.