I see you use Linux. Ubuntu?

It's caused by the Noto font. Ask on Ubuntu forums and you should get some pertinent advice about which font to remove and maybe what to replace it with.

I'm refering to Thunderbird v52.8 (64 bit) and referencing emoticons that appear in the subject pane blocking the view of the subject.

How do I get rid of the emoticons?

It assumed you are using Thunderbird since this is a Thunderbird forum. This is an issue with Linux fonts. The instructions for finding a fix were provided above. Use them.