I am using filter to set labels automatically. But in mail list view, subject was truncated because labels take a place.
So I create this addon to show full length subject and message snippet as a tooltip on list view.
Unexpectedly there was a good side effect that is easy & quick message preview is available on the list without affect to read/unread flag.
Contents in tooltip is depends on Gmail settings "Snippets".
Gmail -> Settings -> General -> Snippets
Tooltip is Firefox default tooltip. So this addon can't customize it. But conbination addon "Stylish" and userstyles "Customize font size for Firefox default tooltip" and "Change tooltip colors and fonts" could customize tooltip by yourself.
Default tooltip delay time is 500 milliseconds. If you feel it too long delay, you can configure it by creating new preference "ui.tooltipDelay". You can get more information from Preference reference below.
Important Information: 2016/03/16
Mozilla is changing a lot of things about add-on development again. So this addon might be stop working in the future.
"The Future of Developing Firefox Add-ons | Mozilla Add-ons Blog"
"Add-ons/developer/communication - MozillaWiki - Upcoming Changes in Add-on Development"https://wiki.mozilla.org/Add-ons/developer/communication#Upcoming_Changes_in_Add-on_Development