www/firefox-esr instead of www/firefox[0-9]*

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

I think the package for the latest ESR release of Firefox should always
be named "firefox-esr", for the following reasons:

- Easy binary upgrades.
- No need to update e.g. bulk build limited lists when a new version
  is released.
- Less surprising for users (other vendors usually use the firefox-esr
  name).

I understand we currently have more than two versions in tree -
I don't think there's any problem with naming older legacy versions
using the numbers scheme.

Is the multiple versions thing the only reason we're not doing this
already? I don't really understand this.