FireFox and Mozilla browsers incorporate a possibly useful feature called "prefetch," but it's not something that's terribly useful unless you're on a dial-up Internet connection and it might well get you in trouble by "prefetching" objectionable material. In particular, the Google search engine provides support for this prefetching behavior - so, if Google decides the most relevant results of one of your searches is an objectionable page and your Firefox browser is set to perform prefetching, you could find yourself in trouble for accessing forbidden content even if you never see the page(s) in question.
The scary thing is this: FireFox has prefetching turned on by default. Most people aren't even aware that the "prefetch" behavior is taking place, much less that they may be leaving interesting tracks in the server logs all day long as the result of Google searches.
Here's the good news: it's easy to turn off this behavior. Pick one of the following approaches:
You can toggle this setting by typing about:config in the FireFox location bar. This will bring up a settings page that lists all your settings. Use the filter function at the top of the page to filter for network.prefetch-next, or simply scroll down until you find the network.prefetch-next setting. Right-click on 'true' and choose "Toggle" to turn off prefetching.
Add or edit this line in the prefs.js file located in your Mozilla profile directory:
So, now you know. Go forth and turn off prefetching, unless you're surfing the Internet over dial-up and actually receive some benefit from the feature. Even on a dial-up account, try comparing your surfing speed with and without this feature - you may find that you'd rather explicitly load pages from search results than have Google give your browser a strong hint that causes it to begin prefetching pages even before you ask for them.