I see people in the support forums all the time asking about backing up their gmail mail with Thunderbird and I really do not understand why they are doing so.
Google offer a direct backup of your Gmail mail, as well as all of your Google data, but it is mail that appears to be the thing people want to back up. Google create a backup MBOX mail format file with your mail which they then compress so it is smaller and offer it to you to download. The MBOX format is used by Evolution, Thunderbird, Mac Mail and lots of other mail clients and is a de facto standard for mail storage and exchange.
This archive file can be unzipped and imported into a mail client at any time, now or in the future.
In the case of Thunderbird, the Import/Export tools will import the MBOX file into Thunderbird for you, without fuss.
Sure there are other ways to backup your gmail, but this is probably the least difficult and most sure of any and uses significantly less bandwidth than any other method to boot.