Yes, it seems an important third step isn't mentioned in that page:

(1) Launch responsive view (Ctrl+Shift+m or the Developer menu)

(2) To choose the desired size in the dimensions control, either

(A) Use the resizing bars to the right and bottom of the content to resize, or (B) Click the text in the dimensions cotnrol and edit to the desired new size

(3) Click the triangle on the right end of the dimensions control and then click "Add Preset" at the bottom to name and save it separately from the "custom" size of the current window

Success?

Note: These are saved in a preference that you can later edit in about:config if you need to streamline the list: devtools.responsiveUI.presets

Thanks. I tried editing and saving, but the responses seem erratic (e.g. sometimes the new size is not saved - i.e. it stays the old size - even after giving it a name/saving). I think devtools.responsiveUI.presets looks pretty easy, so I'll try there later.

Try to press the Enter key after having changed the dimension value to make Firefox show the new setting.

Copied and pasted to devtools.responsiveUI.presets, works. Success!

Just to explain... Opened devtools.responsiveUI.presets, right-clicked Modify, copied string to text editor, in text editor changed values to what I needed, then pasted them back into devtools.responsiveUI.presets