I use this addon to cite text from websites with the page URL. Unfortunately, "Context dependant custom format variables" stopped copying URL's for plain text, after a Firefox update.
I checked on the author's page for the addon: https://evilnickname.github.io/link-text-location-copier/
It offered "Context independant custom format variables" in a second table. I used those. Changing the "plain text" menu format works perfectly (even, at the moment, in Nightly):
%selection% — %N — %pagetitle% — %N — %pageurl%