Import, in general, creates a new folder, ideally under Local Folders. It doesn't offer a merge facilty, but I believe that the folder name shown in Thunderbird is a local label, and the underlying folder name can and will be different. Of course, having two folders with the same name could lead to opportunities for confusion, so I'd take care to avoid such a situation. The general method is to import to a unique folder then merge by moving the contents within Thunderbird. This applies to Address Books too. I've never tried the route you're suggesting. I move my mails incrementally from Gmail folders to Local Folders, which is fine so long as there is no large backlog of messages. Your approach might work well with there being one large file downloaded via your browser. Email protocols are not good with large data sets. However users working with Gmail's mbox files have reported difficulties. There are several flavours of mbox format, and those produced by Gmail may not be what Thunderbird needs.