I solved this problem already by finding this thread previously : https://support.mozilla.org/bm/questions/1399586 If your thunderbird window shrinks when being displayed on your internal laptop monitor but expands when displayed on an external monitor (or vice versa) it is because if the value for ''layout.css.devPixelsPerPx'' (Edit -> Settings -> General -> (all the way down) Config Editor -> (search for) layout.css.devPixelsPerPx) is set to a positive value. It seems like the value has a reverse effect in between internal and external monitors (shrink one and expands one). To fix this issue, set the value for ''layout.css.devPixelsPerPx'' to -1 I am posting this question to display this solution to the public if anyone else experiences a similar issue.