I want to write a complex piece of coding that will do several things:

1) It will clear the contents of a folder named "Cache".
2) It will open (if not already) Mozilla Thunderbird.
3) It will find the latest email that starts with a specific title.
4) It will download the files into two folders: One with today's date and one entitled "Cache".
5) It will open Internet Explorer
6) It will navigate to a specific page.
7) It will access a specific link on the page to download a file.
8) It will download this file into Cache and the folder with today's date.
9) It will open these excel spreadsheets in Cache one at a time and edit the columns to make it importable.
10) It will import the files.

I'm a little overwhelmed with what I'm trying to do. How, if I can, can I have VBA Access access Thunderbird? SimpleMAPI?

since you got all the things in your mind, why not start coding. And if you have doubts whether vba Access should be able to access Thunderbird, why not make a simple code to do it first, so that you'll be able to check the limitations.