Arrows at left of Subject column (left arrows and right arrows): What do they mean?
(These are curved arrows.) They may have to do with the chain of correspondence. Placing the cursor over the arrows does not provide any more information. Nor does searching the Support history in Mozilla Support.
Chosen solution
These arrows are refering to whether you have replied and forwarded the email.
Whilst a few icons have been modified over the years, many are still the same as the info : http://kb.mozillazine.org/Icons_-_Thunderbird_2
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The left arrows are red and the right arrows are blue. (Something went wrong and this important information was left out of my original question.) (Mozilla would not allow me to edit the question!)
Chosen Solution
These arrows are refering to whether you have replied and forwarded the email.
Whilst a few icons have been modified over the years, many are still the same as the info : http://kb.mozillazine.org/Icons_-_Thunderbird_2