In Firefox 48, Mozilla has decided to remove the "Load tabs until selected" option because it's "broken under e10s".
Here's the bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1257554
What this option does is really help startup performance when enabled, by not loading all tabs at once. This is great, but if you don't have a lot of tabs or your machine is powerful enough to handle it, maybe you'd prefer to have everything load at once (like websites that send you notifications and stuff).
This is a really lazy decision on the part of Mozilla in my opinion. Let's sweep the bug under the rug by hiding the option for this broken feature of ours instead of fixing it! Personally, I think this option needs to stay in Options and with e10s getting better and better, loading everything at once might actually be a pleasant experience, and a lot of people might actually prefer to have all their tabs to load at once on startup, with throttling to make it smoother (load 1-2 tabs at a time). That's what I believe Chrome does right now.
Hope Mozilla gets some sense regarding this regression.