Building esr versions of Thunderbird addendum

[jfx2006]

The build documentation applies to comm-central. It will probably work for comm-beta since it's relatively similar to comm-central.

Building an ESR version gets complicated the further out from its initial release. The provided tools like mach bootstrap and --enable-bootstrap don't always work. Official builds are pinned to specific compiler versions and don't really get updated unless necessary. And "bootstrap" mode is not used.

There seems to be a growing number of people rebuilding Thunderbird ESR versions for one reason or another, it would help to have some documentation that doesn't involve "use bootstrap".