Could you tell how you tried to import the mbox file, i.e. did you use any tools or add-ons, did you just copy the file into Thunderbird, or something else? Also, did the import stop, or do you only see 750 messages in Thunderbird even though importing itself appears to have succeeded?

I imported the mbox file using an import/export add-on. I only see 750 messages as it appears the import was successful. I'm unable to see any loading bar/progress bar when importing so I assumed it was successful.

Any advice would be greatly appreciated. Thanks.

If you are referring to the ImportExportTools add-on, make sure to install its latest version from here instead of the page at addons.mozilla.org - that should give you the 3.2.5 version instead of 3.2.4.1 in case you don’t use it already. Its update history however does not show any improvements with regard to importing mbox files.

Also note that at least one similar report (dated March 3, 2017) of email not being imported properly can be found on its review page on AMO, so you may want to consider using another piece of software when 3.2.5 fails as well, such as MailStore Home.

Does this help?