If you use both the online email client Gmail and the desktop email client Thunderbird you might like the idea of porting some of the features and shortcuts that Google has introduced to Gmail into Mozilla Thunderbird. Gmail UI does that. It does not port all of the features to the desktop email client but some of the most popular ones. The add-on for Thunderbird is compatible with all Thunderbird versions except the Thunderbird 3 branch but it is likely that it will be compatible once Thunderbird 3 is officially released.
One of the most interesting features is the archive function. Gmail UI adds the archive function of Gmail to Thunderbird. The user selects a folder name for the archive that can be used on account basis which would mean that the folder had to exist under every user account or one global archive folder which obviously only has to be selected once. The keyboard shortcut [Y] moves selected emails from the current folder directly into the archive folder.

That’s however not the only feature that has been ported over. It is possible to move messages up and down with the [J] and [K] keys, mark emails as junk with [Ctrl J] or focus on the quick search bar with [/]. Expressions are added to the quick search bar which mimics the search expressions in Gmail. Some basic examples would be to enter terms like from:paul, to:mike or f:bob t:dave s:help.
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Heh that was a little anticlimactic. I would have thought surely the most popular feature of the gmail client would be the threaded conversation view (and no, thunderbird’s view does not count unless you are blending sent/received mail).
@adante:
The feature request is here:
https://bugzilla.mozilla.org/show_bug.cgi?id=488044
Please comment on that bug or vote for it, to get the Gmail threading in Thunderbird 3!