Lightning "should" work with either Thunderbird or SeaMonkey, and in 
fact it does — sort of. Every six weeks there is a new XPCOM version, 
and Calendar and Mailer may get out of sync for a few days around that 
time, which is a problem since Lightning is one of the few extensions to 
which the "Default to Compatible" feature is not applicable. This also 
means that, unless I want to run two "mailers" in parallel (one for Mail 
and one for Calendar) there is no choice of versions: I have to use the 
Calendar version which corresponds with my Mailer version — or vice versa.
Sunbird used to be quite lightweight compared to Thunderbird, and — if 
you didn't need email invitations — it was quite useful as a standalone 
program.
If Sunbird cannot be revived for fear of divergence, would it be 
possible to run Lightning on top of xulrunner rather than of 
Thunderbird? Hm, I suppose that if it were, it'd already've been done. 
Or maybe run it on top of a stripped-down version of Thunderbird, with 
the mail & news functions not needed by Lightning not just hidden but 
outright removed at compile time?
I know that since it was abandoned, Sunbird has become more and more 
outdated, yet I keep a precious copy of the last Sunbird build — this 
one (2½ years already!):
User Agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); en-US; 
rv:1.9.1.11pre) Gecko/20100526 Calendar/1.0b2pre
Build ID:   20100526025912
Time and again it has helped me a lot when something — anything — 
prevented Lightning from running correctly on top of my trunk copy of 
SeaMonkey. Yes, its timezone definitions are hopelessly outdated 
("Timezone Definitions for Mozilla Calendar" version 1.2009p, installed 
2010-06-14), but I rarely stray out of my own "Europe/Brussels" timezone 
(CET/CEST), whose definition doesn't change, so that's not really a 
problem. (And when I do, it's usually in order to define a non-repeating 
event in Mozilla Standard Time; that one doesn't change much either.)
If and when Lightning becomes non-binary… yes, we can always dream, 
can't we? Then it would default to compatible, and the worst of my 
troubles with it would vanish instantly. It might even become as 
trouble-free as ChatZilla. Time will tell…
Best regards,
Tony.
-- 
TALL KNIGHT: When you have found the shrubbery, then you must cut down the
              mightiest tree in the forest ... with a herring.
                  "Monty Python and the Holy Grail" PYTHON (MONTY) 
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