Chris Coulson was the maintainer of Firefox packages for Ubuntu back in 2012. He had this to say on the topic of "Why Ubuntu is not using the Firefox ESR". Still relevant, IMHO.
I've noticed that more and more "basic users" (=not computer enthusiasts) have learned the importance of keeping their browser up-to-date, it's been in the news so much. They know how to check the version they are currently running (Help -> About) and can check what is the latest version with a quick google search (or it's mentioned in the news).
I'd hate to be explaining to people I give support to why they are running version 45 when the latest version is 51 (this was the situation before this week's ESR branch update). They may not be computer enthusiasts but they understand numbers.
All of the above assuming Ubuntu MATE is still meant to be "an easily supported Linux distro for family and friends... for not super technical users" (paraphrasing), as Mr Wimpress has stated in interviews. This was the very reason I took a look at Ubuntu MATE in the first place, being in the same situation as him; converting friends and family from Windows to Linux and wanting them to have a stable system that allows them to get stuff done (and minimum amount of support calls for me).
Interview With Ubuntu Mate's Martin Wimpress:
(first 4.5 minutes are about why and how Ubuntu MATE came to be):