A folder containing many messages or other subfolders in Thunderbird is actually implemented as a single file in your Operating System. So it used to be commonplace to say that the upper limit would be set by your OS, typically 2 or 4 GB. However, I understand that this limit has been removed in Thunderbird, though I suspect it means there could be several files involved in storing one large folder. Having said that, even if it's technically possible to have an enormous folder, it may not be very efficient or fast, and for performance reasons, is probably best broken up into a large number of smaller folders (files). I use Thunderbird's Archives feature for just this reason; you batch up stored messages into age-based chunks and thus avoid a large single file that just incrementally increases in size every time a new message arrives or is stored in it. Even if folders represent "fossil" material, they can still be searched. Having messages stored in non-current folders doesn't mean their contents aren't accessible. In short: try to split your material up into smaller topic-based folders, whether by use of Archive folders or something similar. Try to file everything on arrival and keep your Inbox as empty as possible.