Firefox Block images

I use the 'block images from' option in firefox to stop adverts from a
particular url appearing on the page by clicking and holding until the
drop down menu appears.

I love firefox's ability to zap adverts in the way, the problem is the
menu does not always appear. Presumably these adverts have a way of
disabling it to stop you blocking them.

I tied control click but that brings up a different menu.

Is there a way to make the menu come back or stop adverts from disabling it?.

Thanks