I first knew Firefox's special release ESR (Extended Support Release) via an error during system update, which is intentionally for being used in mass deployment of organizations. A few days ago, Blogger dashboard display this message:

Your browser is no longer supported by Blogger. Some parts of Blogger will not work and you may experience problems.
If you are having problems, try Google Chrome. | Dismiss

Aside: I really don't like Google has been prompting Chrome more and more whenever it has a chance, though I can't really blame it since Chrome is its own product.

At first, I believed it's a mistake because I didn't think ESR was out-of-date. But after reading about ESR's proposal, I realized that the releases of ESR weren't what I thought. Originally, I thought their functionality somewhat matches normal releases, but they don't. Every point release (##.0.x) matches the version number of normal release, e.g. Firefox 15 for Firefox ESR 10.0.5, 16 for ESR 10.0.6. There has nothing to do with normal release except the release time. So, Firefox ESR 10.0.x is based on Firefox 10, that means Firefox ESR 10.0.x has only Firefox 10 functionality.

On Gentoo, the latest stable version of Firefox (both source and binary) is Firefox ESR 10, the unstable version is Firefox 13, which is the currently official stable version of Firefox.

If I want to get rid of that message, I can use Google Chrome (from official binary package) or I can upgrade to Firefox 13. But I don't use Chrome and don't like unstable packages.

The message is not annoying to me, just don't exactly know what functionality will not work properly. Since I haven't encountered any problems, I guess I will just live with that message for now.