Yes, but if you add them all explicitly to the addressing box I'd expect you to get an error when you exceed Thunderbird's limit.

http://kb.mozillazine.org/Limits_(Thunderbird)#E-mail_addresses

As it mentions there, if you use a Mailing List you can get round this particular limit.

I am using the mailing list option. I don't receive any errors just complaints from people asking why they are no longer receiving my weekly emails.

Ouch!

have you considered a mail merge? There you send an individual and personalized message to each correspondent. It might be easier to unravel which ones are giving trouble.

I have an order form embedded into my emails that my customers fill out, so I'm not sure how a mail merge would work?

You create a template message with optional placeholders for the addressee's name etc.

No reason why this couldn't be your regular email message, I think.