By default, Firefox loads your home page(s) at startup. You can configure multiple URLs (How to set the home page). However, these are the same URLs loaded in a new window (Ctrl+N) and by the Home button, so this might not be the most convenient feature.

Firefox also can reload pinned tabs at startup, as long as you Quit Firefox with these tabs still open. More info: Pinned Tabs - keep favorite websites open and just a click away.

Thank you, that was really buried.

Note that you can also add URLs to the command line you use to start Firefox via a bash script.

#!/bin/sh

cd <path to firefox installation>
./firefox url1 url2