Recently I have started having a strange error when using Firefox on certain sites. Here's a screenshot...

It looks as though certain site elements are being blocked and a red crossed-out "f" appears instead. This also happens on some entire web pages.
Can anyone tell me what exactly is happening here, and how I can stop it?!
Thanks!

It looks as though certain site elements are being blocked and a red crossed-out "f" appears instead. This also happens on some entire web pages.
Can anyone tell me what exactly is happening here, and how I can stop it?!
Thanks!