On the face of it, this seems desirable, but there are some serious issues with this behavior. If you pay for your bandwidth by the byte — or if you have a bandwidth cap imposed by your ISP — then all that hidden page-fetching by Firefox still counts as downloading to your ISP.
But worse, if you visit a page that contains links to content that your employer finds objectionable or that your government finds illegal, then shouldn't it be your decision whether or not to click on those links? The fact is — unless you are surfing using anonymous free WiFi — your IP address can be tracked directly to you personally, and nearly every page you fetch is logged in some Web server's access log. So how do you turn off this "feature" in Firefox?
It's pretty simple. In the Firefox address bar, type:
about:configand press Enter. If this is the first time you've done this, you'll have to click a button that means you promise to be careful. This brings you to a list of Firefox's internal configuration options. In the Filter field at the top of the page, enter "prefetch". You should see a single line for an option named "network.prefetch-next" that has the value true. Double click the option to change the value to false. That's all there is. Your change takes effect immediately, though I suggest that you close and re-start Firefox just to be safe, because it's possible that this configuration value is only consulted at start-time.