Using the example here:
to set the opening position of a new tab, I use:
browser.browserSettings.newTabPosition.set({value: "afterCurrent"});
as described in the doc. However I get error:
browser.browserSettings is undefined
What should I do?
Ugh, sorry for the noise, didn’t have permissions set.
What permission did you need to set?
If it’d be helpful to add that to the documentation, we can do that. 
browserSettings
It’s actually mentioned in the browserSettings doc:
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/browserSettings
but not in the browserSettings.newTabPosition doc:
which is understandable, as I suppose they expect devs to have read the first one. It might be convenient to have it in both, but that could open a floodgate as I expect it is not normally done that way.
Thanks for enquiring.