Is there a mail forwarding feature where Email1 receives a mail and automatically sends to Email2?

You do this at your providers server. You can do it in Thunderbird with filters but what is the point of bringing the message into Thunderbird only to send it out again? If using filters in Thunderbird you would need your computer on with Thunderbird running. Log into your account at the providers webmail site and do it there.

I'm asking because the webmail/server does not have an email forwarding feature.

Here's what I was trying to do: Two emails logged in in Thunderbird, Email1 and Email2. Email2 should now be the default email due to company's new domain. And of course, Email1 will still receive emails because not everyone will be aware of the change instantly. So I was asking if Thunderbird has a feature where Email1 can automatically forward emails to Email2.

That's the point of my inquiry.

So you have both email accounts in front of you in Thunderbird. What is the point to forward the messages out and back to the new address? Just drag and drop the messages to the new account if all you want is them in the same folder.

Or create a filter to move them. https://support.mozilla.org/en-US/kb/organize-your-messages-using-filters