What does the + button do?

The paragraph below from mozilla.org mentions clicking the + button to create a new field to enter the new address. (Complete help topic here: https://support.mozilla.org/en-US/kb/blocking-sender)

The Message filter that you have created can be opened and edited by selecting the Message filters... entry in the Tools menu. It is not necessary to create an individual filter for each address you want to block. Now you have the Block list filter, the next time you want to block someone you can add the person's address into the top of the filter by clicking the + button and thus creating a new field to enter the new address.

The + button can be clearly seen on the pictures shown in the linked article. How does your filter dialog differ?

Can you make it bigger, in case the buttons are outside the visible part of the display?

Are you using a theme that might be interfering with the display of the filter dialog?