Thunderbird is causing my laptop to run hot

[Phil]

Top shows CPU usage to be better than 80% and of course the laptop 
immediately runs cooler once Thunderbird is killed. Is anyone else 
having the same problem? Thunderbird has been my favorite e-mail client 
for many years, perhaps I should consider a lighter e-mail client? I 
have used Claws, the default e-mail client for the Raspberry Pi. It is 
definitely light weight but of course it's not as feature rich as 
Thunderbird.

Firefox is just as bad, and at times even worse, making the laptop very 
sluggish if they're both used at the same time. I have used Chromium, 
maybe I should have another look at it? If it matters, I use XFCE as my 
desktop manager.

-- 
Regards,
Phil