Probably what's being forced on them at work. That's generally the only place ESR is used. It's actually all it's meant for (large scale corporate/organization rollouts where they need to test and peg a specific version for use with web apps).

Though you can have Firefox ESR installed and use it as your primary while still having a copy of Firefox Portable handy for other user on a flash drive or even right in My Documents.

I use it to avoid Australis (but still getting security updates).

You don't find themes like Simple White¹ or FT DeepDark² to be acceptable?

1: https://addons.mozilla.org/en-US/firefox/addon/simplewhite/

2: https://addons.mozilla.org/en-US/firefox/addon/ft-deepdark/

The themes don't change the arrangement of the UI.

Try Debian if you need something so rock solid that you can only get security updates.

I do use Debian for my desktop. So? I still have Australis. The themes don't change that fact.

If you're on Windows or Linux try Palemoon [0].

[0]: http://www.palemoon.org/