The message filters in Thunderbird can be found in the main menu under Tools.

But this would probably be best done at the server. Log in to your account at your email provider's website and look for filters or rules options there.

Such filters can sometimes be counterintuitive. I found I had to make a copy for myself then forward the original to the alternative account.

I set the filter in TB to have emails forwarded from a secondary comcast account to my primary comcast account. I'm not sure if I didn't set the filters correctly or if I need another solution. Here's how I set the filters:

1. Apply filters when getting new mail; 2. Match any of the following & then listed To, From, CC followed by the secondary email address that I want to automatically forward emails from; 3. Under Perform these Actions, selected Forward To and the primary email address that I want to receive all forwarded emails

What am I doing wrong?

I don't know why it's not working. But I would be unhappy with the results; all those forwarded messages arriving in the primary account would now appear to have come from your secondary account, and not from the original senders.

If you do it as I suggested, and get the forwarding done at the server, the true "from" provenance is usually preserved.