Firefox ESR is an older version of Firefox that continues to receive security updates. It is made for conservative enterprise environments that care more about stability (as in: not changing) than features or fixes. It may or may not be more stable (ideally, newer versions would fix stability issues in older ESR versions), and may use more or less memory (regressions in memory usage should be reported).

I have tried it in the sense that I have a copy around for testing in those odd cases where someone is using ESR. I don’t recommend it.

I use Firefox ESR on my Debian 12 machine and Firefox 114 on my Arch machine. I haven’t noticed a massive amount of difference between the 2 in my day to day.

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 I do occasionally get flagged by websites for using an out of date release, which is kind of annoying.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.