What do you think is best?

  1) keep Thunderbird 1.5.0.x in mail/thunderbird and add Thunderbird 2.x as
     mail/thunderbird2 (like I did with Firefox 1.5.x vs 2.x)

  2) update the existing mail/thunderbird package to 2.x (like I did with
     Seamonkey 1.0.x vs 1.1.x, which was a similar upgrade)


There are three considerations:

1) please make sure that a user with an installed thunderbird who
   updates pkgsrc and does either pkg_chk -u or pkg_rolling-replace will
   have something reasonable happen.  Reasonable choices are staying on
   1.5 or going to 2.  Unreasonable behaviors are failing to update
   because the old pkg name is no longer a pkg name.

   The right fix is to have a rename database, but we don't.  This
   argues for having thunderbird just updated to 2, so that people don't
   have 2 and then have to update to mail/thunderbird (which will be 2),
   which won't go smoothly.

2) Can we say, and really mean it, that everyone should simply switch to
   thunderbird 2 right now?  Is that the official advice from
   thunderbird people?  If not, that argues for calling it thunderbird2.

3) If switching to thunderbird2 is the standard approach, would people
   need to have thunderbird 1.5?  If so, then the current package should
   be put into thunderbird1 so that people can run it if need be.


So all in all I think it's likely that your option 2, with possibly
reimporting thunderbird 1.5 as thunderbird1, is the way to go.