Rated 5 out of 5 stars

this is something i've thought about writing for a while. it would be close to perfect, given:1. i believe it's against convention to take up a panel in Options, and it's worse if there's no icon.2. it should be clear if Date or Received is used.3. imo, the Date Format pref could be added as a tab in Options->Display; the column name doesn't need its own tab.4. the panel/tab could be automatically selected in the Options window when clicking options in addons manager (it's possible programatically).5. you don't need a new custom column, but can instead use the same method to override an existing column, and let the user decide to which columns the date format should apply.

[followup:] all you do is use dateCol or receivedCol in addColumnHandler(). but you need to get the dates like this: _fetchDate: function(aMsgHdr, aName) { let date = aName == "dateCol" ? aMsgHdr.date : aName == "receivedCol" ? aMsgHdr.getUint32Property("dateReceived") * 1000000 : null; return date; },

[followup2:] the 2 options panels *really* need to be combined into 1... also, i think there is a better way than listbox (grid?) to display the format symbols help box; the text should especially be copy/select enabled (readonly textbox?). then it would be perfect. ;) 4->5

This review is for a previous version of the add-on (1.0). 

Thanks for your input.
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1. Yes, I've moved it into Options > Display in the latest code.
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2. I've updated the add-on page.
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3. Yes, I've moved it into Options > Display in the latest code.
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4. I'll look into this.
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5. That one thing I wanted to do, but couldn't figure out how. However, if people would like to create multiple date fields, each with their own formatting, then I'm open to that as well.
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Follow-up #1
Thanks for the code. As of version 1.1 the date format is applied directly to the "Date" and/or "Received" columns, and the new custom column has been removed.