IE 11 windows update, mendatory for FF users?

I'd install the update.

Internet Explorer is not a standalone piece of software like Firefox or Chrome. It is a part of the Windows OS. Which is why it is a massive headache as well as a security nightmare. You are perfectly within your right to not use it, but you cannot predict when Windows will decide a part of it must run whether you know it or not. Be aware that even if you disable IE it may affect other software. (I learned that the hard way after trying to install Corel Draw. I found out the Corel installer uses IE to display the EULA. Just one example of why IE should be updated even if you don't use the browser feature.)

I agree.
Install the latest version of Internet Explorer supported by your OS and keep it fully updated.
IE components are a part of the OS so it makes perfect sense for other components of Windows to use them when appropriate. They were designed for this. For the same reasons many third party applications use IE components. Sometimes this usage is obvious, sometimes not. Some applications will explicitly use IE rather than the default browser. IE is well known and understood by developers and using it will often be easier and produce more predictable results than trying to be compatible with IE, and Chrome, and Firefox, and Opera, etc, etc.

You may not use IE but the probabilities are high that it's components or the browser itself will be used by other software. For best stability and security IE should be the latest and updated version available. To do otherwise could put your computer at risk.

I agree with marsmimar and LMiller7.

I'll offer another reason to keep IE up to date:
You never know when you might want/need a second browser. Firefox might get so messed up that you need IE to find/download the solution. Sure, you could use another computer and transfer files; but why not just keep IE up to date?