Xorg's CPU load is only high if some part of the Thunderbird / Firefox window is being drawn; if you run top in a terminal window 'always on top', then minimise and maximise Thunderbird while a big email is downloading you can see Xorg's CPU load going from high (50-70%) to low/normal (10-15%). You can also observe this behaviour by maximising another window in front of Thunderbird.
Xorg's CPU load is only high if some part of the Thunderbird / Firefox window is being drawn; if you run top in a terminal window 'always on top', then minimise and maximise Thunderbird while a big email is downloading you can see Xorg's CPU load going from high (50-70%) to low/normal (10-15%). You can also observe this behaviour by maximising another window in front of Thunderbird.