Here are a few workarounds:

• Change your mailbox settings (for that specific account) so that all your emails are also copied to a local folder as they arrive (check 'Copies & Folders'). If you are a user of those email providers which regularly lock their users out of their accounts because they refuse to give a telephone number (or their dob) or have "detected unusual activity" (typical harassment routine for us, "outlier" users) then you must have done this already. This entails downloading thousands of emails to your local disk though once you tick that option. Not ideal if you are on a slow connection or have some kind of quota. Once you do it then you will be able to see your emails in folder you mentioned in a single file. It will probably be the biggest there.
• In some Thunderbirds, there is a 'Quick Filter' button for each mailbox (this is on the left of 'Search' but it is not the 'Search'). Use that to filter your desired emails. In the Display you will now have only the emails you filtered. Select them all (ctrl-a), right-click and 'Save as' into a new temporary folder. Each email will be saved into that folder in .eml format for further processing using Perl.
• Create a MailExtension plugin. It's javascript. Plugins are distributed as xpi files which are just zip archives. You can inspect a simple one here, or use this tutorial.
• There are a couple of Thunderbird extensions for creating mail statistics. See this or this.

So, plenty of options to keep you in front of your computer during the festive season (wasn't that what you were looking for really? hehe)

bw, bliako