...is another man's bug.
Since Thunderbird 3, if you selected part of a mail then hit reply, only your selected part gets quoted. Incredibly annoying for those of us who think out loud with our mouse.
But it can be switched off. Edit|Preferences, Advanced tab, Config Editor. Then in the search box type:
Change it from true to false (double-click that line is enough).
It turns out this was actually a Thunderbird feature request, dating from 2000! https://bugzilla.mozilla.org/show_bug.cgi?id=23394