Is this the article you're talking about? https://support.mozilla.org/en-US/kb/blocking-sender

For the 'plus button' look at the bottom screenshot.

Please walk me through the steps to add an email to the Block List, which I have created. Many thanks

Can you describe what you did so far and at which point you get stuck in the article?
Please be specific.

I have set up the Block List filter. How do I add additional addresses to the filter? I assume I pick the email and then what do I do. Do I right click on the address of the sender? Where do I locate the + button? Is it on the key pad or the email? Please advise. Many thanks!

The plus button is clearly displayed in the screen shot above. It is a button on the filter dialog box.

Still confused! I click on unwanted email. Then rt. click on its address and then on Create Filter from.... and the address shows in the 3rd box. I type Block List in Filter Name and click on+ button next to address. Only thing that happens is a line of more boxes appear: From, Is, Address + -. What am I doing wrong? I get a lot of scam each day that I would love to block. If you like, please call me at 252 945-0312 and walk me through it. Many thanks!!!

I get a lot of scam each day that I would love to block.

You started this thread with 'blocking a sender'. Now you're talking about 'scam'.

There is a fundamental difference in blocking messages from a specific sender or just blocking spam messages.

So what do you really want? Please be specific.

I want to block messages from specific senders which would include unsolicited and undesired emails.

Good.

I have set up the Block List filter.

Can you post a screenshot of what you've set-up already, similar to the bottom screenshot in the article https://support.mozilla.org/en-US/kb/blocking-sender ?

http://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem

I have set up Block List but don't know how to open it. What am I doing wrong? When I get an unwanted or unsolicited email I want to stop future emails from them. Upon receipt I click on address and click on Creat Filter from.... and the address shows in the 3rd box. I type in Block List in Filter Name and click on + button next to address. More blocks appear:From, is, address, +, and -. What am I doing wrong. Please walk me through the proper procedure. Thanks!!!

Tools menu (Alt+T) > Message filters. Select your block list and add the new email address.