You can reset to the original list using the about:config preferences editor, or you can edit in the missing presets. Note: since it is JSON, it's a bit messy to edit, but... it still might be faster than re-adding them by hand.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste responsive and pause while the list is filtered

(3) Right-click the devtools.responsiveUI.presets preference and Copy Value, then paste somewhere for reference if needed -- note: this preference only exists if you have customized the presets

(4) Double-click the devtools.responsiveUI.presets preference to open a dialog to edit it, delete the existing value, and paste the following (it's one long string), then click OK:

[{"key":"320x480","width":320,"height":480},{"key":"360x640","width":360,"height":640},{"key":"768x1024","width":768,"height":1024},{"key":"800x1280","width":800,"height":1280},{"key":"980x1280","width":980,"height":1280},{"key":"1280x600","width":1280,"height":600},{"key":"1920x900","width":1920,"height":900}]

Note: That is the default on my Firefox -- discovered by adding a new DELETE preset so that Firefox adds the devtools.responsiveUI.presets preference, and then removing DELETE to leave just the defaults.

(5) If desired, edit the preference to add additional comma separated resolutions.

Thank you, sir. Pasting the presets worked great. I'm back in business.