It usually means the message has been replied to. Its appearance (or failure to appear) may hinge on the Theme being used. You may have different themes in your two installations.

Additionally, I have noticed that in menu items where there needs to be a marker to tell you which option has been selected (typically a checkbox or radio button), these are absent in some Linux installations. Developers looking at this have commented that it's connected to which version of GTK is installed. Possibly your missing icons are part of this problem.

Actually, it is not the arrow for replies. It is on all my email entries in my W10. They are just straight arrows pointing left and are a light gray color. They are in the Correspondents column.

johnfaye

Ah, those arrows!

See this discussion: https://support.mozilla.org/en-US/questions/1161351