User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:65.0) Gecko/20100101 Firefox/65.0
Steps to reproduce:
In Preferences > Home in the New Windows and Tabs section, the first entry is "Home page and new windows" which allows choosing Firefox Home (Default), Custom URLs..., and Blank Page. The next entry is New tabs where the choices are only Firefox Home (Default) and Blank Page. Firefox should allow Custom URL... for new tabs like it does for new windows.
There's no way to specify a custom URL to be opened in new tabs.
There should be a way to specify a custom URL to open in a new tab like there is for a new window.