Firefox ESR is basically designed to prevent rapid upgrade cycles in browser breaking your software, since you cannot be constantly testing the latest version that comes out (pretty quickly nowadays).
The way they do this is that they simply update the user browser only once a year. With a 12 week overlap between two version so you can test if anything is broken. However, security updates continue throughout the year.
There is no additional difference from the standard desktop browser. Quote : “Firefox ESR is functionally identical to Firefox for desktop,”
So if it works in Firefox. It works in ESR.
The support gets no special treatment from Mozilla. It is via a mailing list: https://mail.mozilla.org/listinfo/enterprise Which is just a group of IT guys like us. The only difference is that everyone is using the same version of browser so if something is broken for one guy due to some browser issue, others can easily test it. You don’t need to ask “which version of Firefox are you using?”
Download is available here : http://www.mozilla.org/en-US/firefox/organizations/all.html .
Warning it overwrites the Firefox you currently have installed, since it is exactly the same. My settings and addons e.g. firebug were carried over without issues. Current ESR version (April 2013) is equivalent to Firefox 17 (which was the latest standard version Nov 2012). After you install this all it effectively does is change your update channel to esr: