The error is because Firefox does not know what and how to display that content.
For example, let us take a flash file. If you go to a webpage that displays flash and if you do not have the flash player plug-in installed then Mozilla Firefox will not know how to display the flash content in that page and hence the region where the flash file is to be displayed contains an error message as stated above.
So incase, you get this kind of error, mostly it is due to missing flash players. So get a flash player plug-in and install it in your system. This error can also occur due to other missing plugins which come default with Firefox.
If it is not a flash player that is missing, then you will have to reinstall Firefox.