ESR is a feature freeze version. So there will be much fewer changes and, thus, fewer updates and fewer new bugs which need updating as well. However, it will get all the same security updates as mainline, if they also affect the ESR version.
So, overall, there will be fewer updates.
As for complaining about getting update notifications, if you are using Linux with a distro package, it will get updated easily through that and there will be no notifications inside Firefox, itself. I have no idea about MS-Windows or other platforms. If you don't want to update, then set a policy to turn off updates (applies to either mainline or ESR). In Linux, put:
{
"policies": {
"DisableAppUpdate": true
}
}
into $INSTALLDIR/distribution
There is something similar for MS-Windows registry stuff, I believe.