Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1b3pre) Gecko/20090117 Lightning/1.0pre Shredder/3.0b2pre This must have regressed in the last day or two, I noticed related bugmail doesn't group by topic anymore though I've set it to do so previously. Sid0 sees this on Vista too. (privatised comment #0, wrong screenshot there, see comment #1 for the correct one, and the description is here)
I see old, already previously grouped bugmail, being grouped. New groups don't seem to be formed, however.
see bug 473439 and friends
Setting the prefs as described in bug 473439 comment 11 indeed fixed it.