The Mozilla Thunderbird e-mail client doesn’t provide a back-up tool. It is possible though to use the search function to select only those e-mail messages that are more recent than your last back-up and save them, but this process doesn’t store the folder structure that your messages were in. This is only a working solution if you use only a few folders or none at all. I recently had to restore from a hard disk crash and spent half a day putting e-mails back in folders. (Another option would have been to keep my restored back-up in in its own folder; after all, it’s mostly old mail.)
Looking for back-up tools for Thunderbird, the official Mozilla.org recommendation appears to be to get a program called MozBackup. The advantage of MozBackup is that it creates back-ups of profiles for several Mozilla tools, not just Thunderbird. Unfortunately it doesn’t do incremental back-ups.
Looking through mozilla.org’s support forums I encounter mostly disbelief that anybody would want to do incremental back-ups of e-mail messages and folders. As a web developer I often get sent large files by clients and designers. Lately this has decreased a lot—people store their files in the cloud and send me a link—but it still happens. Before the crash I had about 10 GB in e-mail on my hard disk.
There is a back-up plugin (I believe Mozilla uses the word add-on instead of plug-in) called ImportExport Tools which I think I used before to migrate from Pegasus to Thunderbird but which you can also use for back-ups. (Strangely the plug-in is stored under Miscellaneous at addons.mozilla.org instead of Import/Export.) This contains an option for structured storage, that is to say, storing mails with their respective folders. Unfortunately this option won’t let you select the specific mails you want to export, it only works for all mail at once. It does have an automatic back-up setting which may or may not store the folder structure. I will update this posting once I find out more.
Note also that the author of the plug-in reports that storing folders is at the moment unstable, i.e. may not work.
Back-ups are dangerous if you don’t test whether or not you are able to restore them. As far as the ImportExport Tools plug-in is concerned I can report success in this regard. I created a second account in Thunderbird and imported the back-up of my primary account there. I only performed spot checks, I did not check if all mails were recovered.
Another option would be to store e-mail without attachments. That way I could easily store every mail I ever received on a single CD-R, which would make the whole incremental back-up thing just a little less important. Of course you’d have to find a way to store your attachments first. In case you are using your e-mail client as a sort of document management system this won’t work. Let’s say you receive lots of photos of loved ones via e-mail and then when you want to see these photos again later you view them in your mail client. In this example you never bothered to store your attachments in a separate location, so back-ups without attachments would be less of an option.