I use it because that’s what my Linux distro packages. It’s fine, but unless you’re averse to change there really isn’t any reason for a regular Firefox user to choose ESR over the regular release. I do occasionally get flagged by websites for using an out of date release, which is kind of annoying.

I use it on one machine because it’s the only thing available there. I frequently have problems with web sites that refuse to work because my “web browser is out of date.” Google, in particular, is bad about it, and even seems to actively user agent mocking from working. I know that’s not FF’s problem, but it is a severe issue that makes me wish I could update to a newer version.