How to make Thunderbird play nice with Google mail

Solution 1:

There is no need for a workaround anymore. Thunderbird version 38 and newer support GMail labels. According to the documentation an e-mail is downloaded only once:

Note that a message can have multiple labels (for instance, "Personal", "Travel", "All Mail" and "Starred"). In this case, a single copy of this message will be downloaded, but it will be displayed in all the corresponding Thunderbird folders.

Solution 2:

For the record, IMAP does support tags (they're called keywords and implemented as flags), and Mozilla Thunderbird and SeaMonkey use IMAP tags.

See this discussion from 2006 with Mark Crispin.