When reading web mail I get an annoying pop-up asking me whether to "allow" the page, each time I move from one message to the next. How can I stop this?

Just under the toobar a black bar appears, on the left it says "Firefox prevented this page from automatically loading", on the right is a button, "Allow" and an X to make the bar go away. However, the next message in my email list on which I click causes the bar to reappear, so I have to repeatedly "allow" or X-out the bar. Very annoying.