Blocking sites with FireFox
 

How do you block sites using FireFox? Do you have to get some kind of plugin or something to do that or do you just go the options menu or something?

Define "blocking sites". What are you trying to do?

I did a little digging around and found this

https://addons.mozilla.org/en-US/firefox/addon/3145

It's an add-on that allows you to block the sites of your choosing.

I'm going to assume you mean that you want to block access to sites, so that users may not navigate to them. You basically want to filter out that which you find to be undesirable or dangerous. If I'm wrong, tell me.

I'd start using OpenDNS, they have a site blocking facility. Doing it with Firefox is pointless because the user can always switch to IE, Opera or some other browser. Doing it at the DNS level is much better, because that applies to any browser.

Or you can add an entry in the hosts file (C:\windows\system32\drivers\etc\hosts)

Cool. I'll use this thing. Thanks.

Do you not care that anyone with minimal computer skills can bypass that without breaking a sweat? If the objective is to stop little Jimmy downloading porn, it's going to be about as effective as a fence made out of damp tissue paper would be at keeping lions safely contained.

I was going to pull a prank on this one idiot at school. I would block a ton of his favorite sites and the only browser he uses is Firefox. Yeah, he's that stupid.

Here is a better prank you can pull.
Computer Prank: URL Redirect Trick Video

Much trickier to track down. >:3

Oh well... as long as it's not important that the sites be reliably blocked, and you're certain that the user is pretty dumb, that plugin would work. In fact, it's better than my suggestion for the purpose you have in mind. My suggestion is more a long-term strategy for home use. I somehow got it into my head you were looking to filter either porn sites for younger users, or maybe warez sites for unwise users.