Can someone else access Web sites through my computer?

First off, I believe you. I can think of a couple of ways that you can end up with sites in your browser history though you never actually went to them, or, to be more accurate, were never aware that you were visiting.

The most likely explanation is that there are pop-under advertising windows from those sites being triggered by one or more of the sites you regularly visit. That's one thing I really love about Firefox: instead of just randomly popping up windows from sites you visit, it shows you a warning message.

Other Web browsers have various abilities to do this too, but most of them just silently block things without letting you know (for example, Safari) which is a bit less fun than having the browser tell you that it's helping you not be overwhelmed by pop-ups.

Unfortunately, they don't always work, even the slick Firefox blocker. For example, visit http://www.w3exit.com/ and its demo page and I bet you'll get one of these annoying pop-under ads showing up.

Those Web pages are dutifully logged in the history file and later you'll look at the list of pages you've visited and find that there are alien invaders, just as you are experiencing. That they're porn or scam sites, well, that speaks more to the denizens of the social networks and sites you're visiting.

How do these pages show up without you ever seeing them? They show up underneath your current window, and if you never close the top window to see what's behind it, they can pile up, dozens at a time, and then all silently vanish when you quit your browser, without ever showing their ugly face.

So now you know. No reason to be paranoid about hackers breaking into your computer to utilize your Web browser. The explanation is - most likely - far more mundane.