Spreading emails between several files is less risky.
Opening smaller files is quicker and easier than large ones.
Good advise on how to maintain a healthy Thunderbird: Anti-virus products are notorious for not being able to understand that the mbox file contains several emails and not only one. If an Anti-virus product scanned the 'Inbox' file and thought something was wrong with a small section, you risk the entire file being lost. Hence, do not allow mbox files to get too large, especially ones wchich get a lot of activity like the Inbox. When compacting the select Folder Pane 'folder' which you now know is really an mbox file, a copy is created for this purpose, this requires some free space, but if the Inbox file has become too large it can cause an issue as not enough space is available. If you do not compact on a regular basis to remove old marked as deleted emails, the file might suffer corruption due to poor maintenance and you risk losing everything. If the Inbox file gets too large it is not so easy to open it and fix because most text editor programs expect smaller size. The 'Inbox' mbox file is the most used most accessed gets the most activity moving, deleteing etc than any other file. Note that I'm talking about an mbox 'File' not a 'Folder'. The act of compacting will scan through the document and remove each 'marked as deleted' paragraph and then close up the gap, so everything still reads ok.
So imagine you have various 'paragraphs' which you no longer want throughout that document.
It is 'marked as deleted' and hidden from view, but at this point it is still in that file. When you delete an email, it is not removed. So imagine it like one long text document where each email is like a new paragraph.
'Inbox' is a single simple text file and each time you download an email, it is written to that file, one after the other in the order downloaded, so the oldest is at the top and newest at the bottom. When emails are downloaded they are written to an mbox file called 'Inbox'.