When X11 dpi is set to 115 dpi (or values around it), then rendering of fonts in the windows is broken. All texts are shifted down in the lines, and bottom of the characters is cut. It seems to me that the problem occur in all dpi settings, but in other values like 96 dpi, or 127 the "window" for text line is brighter, and characters don't move out of it, and therefore it is not easy to detect it.
Created attachment 96900 [details] Screenshot from Thunderbird Ther is screenshot from thunderbird, where problem can be observed. This problem I have observed first time around version 0.90, on different machine wit x86, and now with amd64.
Well, so don't set such broken DPI. :) Now seriously, you should take this upstream. Works fine here when DPI matches the system one. https://bugzilla.mozilla.org/enter_bug.cgi?product=Thunderbird
Hmmm. My notebook have 115 dpi screen, and that's problem.
Ok. I have created bug report in bugzilla on Mozilla.org, so this one can be closed