Thunderbird uses the term Junk. Its junk filters do not consider email addresses and have no way of marking an email address as such. It does not filter spam. It does not mark messages or addresses as spam. It does not use a spam folder. Your email provider might or any spam software you are running g.

In my inbox, I can press on a fire symbol next to the email, which moves the email from inbox folder to the spam folder. Then it automatically places these emails in the spam folder and I can not reverse this.

OK. As stated Thunderbird does not place anything in a spam folder. A junk folder,yes, but not spam. Here are instructions for Thunderbird junk controls. Junk not spam. http://kb.mozillazine.org/Junk_Mail_Controls

I have a spam folder. Please see attached photo of my spam folder (circled) and the arrow pointing towards the fire symbol at the email. I accidentally pressed this symbol on an email address and it sent it to my spam folder. Now it automatically puts it there everytime they send me an email. I want to change this from happening and I can't seem to find out how.

Your providers server creates folders that it uses. When configured for IMAP email service you can see those folders. It does not mean Thunderbird uses them. Spam is one of those folders. Thunderbird deliberately uses the term junk to differentiate itself from whoever uses spam but people still cannot grasp the concept. If you go to your Spam folder and click the flame icon what happens? Can you move it back to the Inbox by drag and drop? If so does it stay there?