For ad blocking, I recommend using the uBlock Origin extension.
The add-on asks for permission to Access your data for sites in the mail.yahoo.com domain and Access your data for mail.yahoo.com, but this add-on does not read any of your emails - it only hides (does not display) the advertisements.
You can also view the code by right-clicking the "Add to Firefox" button and selecting "Save Link As...". Then change the file's extension from .xpi to .zip and extract the file.
