firefox60 and thunderbird60 EOL

[nia <nia%NetBSD.org@localhost>]

Firefox 60's time has come. We're at the end of the release
cycle for Firefox 68, its successor, which I've mostly been
maintaining so far. A new extended support release will
be branched soon. If firefox60 is kept we will then have 
five different branches of Firefox in HEAD.

Thunderbird 60 is no longer supported, and there will be no more
extended support releases of Thunderbird.

Thunderbird has moved to a new release model, there are now
no more "extended support release" branches. Rather, the
main branch now has extended support.

Unless there's strong objections, I'll drop both firefox60
and thunderbird60.

Seamonkey is based on the Firefox 60 codebase, so I'll make
it the primary holder of mozilla-common.mk. If you have an
extremely weird web page that only renders in firefox60,
seamonkey likely works.

The last branches without a rust dependency will remian,
Firefox 52 and Thunderbird 52, for reasons that have been
discussed to death. :)