Hi Dan, sorry about the experience you've had.
The Settings page should be working properly, even in this version of Nightly (I just double-checked). Could you try restarting Firefox, in case that helps at all?
The share dialog opening in a new tab is an unfortunate issue we're aware of, and is specific to our Firefox extension (there's no such issue in our other browser extensions). We have plans to fix it, I also personally prefer the overlay so I know how you feel.
We're glad to have you with us, and I hope Buffer still proves useful for you despite these issues while we work on fixing them; if not and you'd like to reconsider your purchase, feel free to reach out to us at email@example.com, we're always happy to help as much as we can.