The limit for "bulk" mail is set by your Mail provider.

No, that was not my question. I'll repeat it here:

What are some good "add-ons" so I can modify Thunderbird to be able to use it to send out bulk emails?

What I meant was that its not TB that limits your bulk-mail. TB is able, but check this question https://support.mozilla.org/en-US/questions/1148301

It wasn't the limitations I needed to know about, it was how to set it up to work with bulk - a lot of addresses (being able to program it in with a spreadsheet of addresses, sending out a lot of emails with a minimum/efficiency of actions, etc etc.

It does look like you inadvertently were able to lead me to the answer however. In the article they did give me the information I was looking for - turns out the add-on is called "Mail Merge". THAT is what I needed to know.

Yes I saw that, but instead of repeating it ....