Wikipedia has to update articles and maintain the server backend

Firstly, updating the articles is the one thing Wikipedia doesn't do, the army of unpaid volunteers does that.

But as for "just maintaining the backend", the Wikimedia Foundation does far more than that. It created and maintains and constantly iterates a huge pile of ever-complexifying frontend code - the wiki itself, discussion software, media tools etc - not just for Wikipedia but for a whole bunch of peer sites. Much of it is pretty cutting-edge, it's used daily by many thousands of editors and there's also the accessibility requirement. I know from personal experience that there's nothing harder than front-end when you have to tick the accessibility box. No doubt Firefox's technical challenge is greater but really the difference is not night and day.