Safe browsing

If you want a safe, private, ad-free browsing experience, then in my mind, you can't do better than Firefox with a few plugins.

Some plugins give you benefit without any need for action on your part. Some however require some input from yourself, and may take a little while to get working properly with the sites you visit.

AdBlock Plus

Benefit: 10
Effort: 0
This is an absolute no-brainer. I can't understand how people can surf with ads. When I use someone's computer who hasn't got an ad-blocker installed, it's like going back in time.
Go and get it now.

HTTPS Everywhere

Benefit: 10
Effort: 0
HTTPS is what keeps your browsing safe and secure, so that only you and the site you're visiting can tell what you're up to. A lot of links don't link to the https version of a site, so this plugin has a list of major sites that support https, and will automatically send you to the secure version.
Get it from eff.org

FlashBlock

Benefit: 7
Effort: 1
This will replace any Flash with an icon you can click. This means videos won't auto-play, and software that tracks you through Flash "cookies" won't work.
Go and get it now

Those first three plugins are pretty much no-brainers. There's not really a good reason not to install them.
The next two make browsing safer, and private, but do require a little training before the results are good.

RequestPolicy

Benefit: 7
Effort: 5
I don't know if you know, but when you visit one website, your browser might load 5, 10, 20 things from other websites in the background. This means that those 20 websites all know that you visited the original website, and if they're on lots of websites, they can build a profile of you.
If you press F12, and click on Network, you'll see all the requests next time you load a page.
For instance - the Facebook "Like" button on websites? Facebook knows you've been to every website that has that on it. Lots of sites use Google in the background too, so even if you never visit any Google sites, and don't use Google at all, Google knows all about you.
RequestPolicy controls what resources from other sites your browser will load. By default, it blocks all of them.
This does mean however, that sites might look strange, or not work properly until you've allowed the resource that they need. It doesn't take long though to get your normal websites set up, and once that's done, you can be sure that your browser is only talking to the websites you want it to.
Install it, and if you don't like it, you can simply un-install it.
Get it here

NoScript

Benefit: 5
Effort: 6
Javascript make web pages interactive. I personally don't like my computer doing anything I didn't ask for, and for this reason I run NoScript, which blocks all Javascript on pages unless I've whitelisted it.
Like RequestPolicy above, it can make pages look strange, or not work correctly at first until it's trained.
Install it, and if you don't like it, you can simply un-install it.
Get it here

Think I'm wrong/misguided/have left out an important plugin? Then leave a comment.