(I am a longtime Ubuntu user but first time Ubuntu MATE user)
I installed Ubuntu MATE 18.04 on a separate partition.
After installing and configuring Thunderbird I see Huge icons/emoticons in the message listing pane for some e-mails. (see attachments)
For comparison I have added a similar screenshot made on Ubuntu(Unity) 17.10. In this screenshot you can see the icons/emoticons with the, expected, normal size.
Both, Ubuntu MATE 18.04 and Ubuntu/Unity 17.10 are running Thunderbird 52.6.0(64-bit)
Is this a problem I can solve by myself? I couldn't find any setting to correct the icon/emoticon size.