You might not be knowing but all Mozilla based web-browser's including Firefox prefetch websites and images that you might be visiting next, for faster loading times as instructed by webmasters, prefetching is enabled by default and webmasters can make the web-browser download webpages and images in-advance (including third-party content) using rel="prefetch" in their code.

You might want to disable this behavior if your ISP charges you pay-per-byte for network bandwidth or you are privacy conscious and don't want to get cookies/images from websites that you yourself never actually visited, to disable simply follow the instructions below:

1. Type about:config in your Firefox address bar and press Enter.
2. Type network.prefetch-next in filter text box.
3. Change the value to false as shown below.