I agree with Ethel. The best way to find out about Firefox is to try it. It doesn't conflict with Internet Explorer, so you can have both on your PC simultaneously. (You can have separate windows open, in both IE & FF, at the same time if you like).
Firefox has always been more secure than IE but IE7 has, belatedly, addressed many of the issues which FF tackled ages ago.
As Ethel says, FF is easily customised to work exactly as you want it to. However, many people (myself included) are perfectly happy with the basic program.
The default settings in Firefox include a pop-up blocker and a spell-checker (for posts like this one). The default spell-checker uses a US English dictionary but you can download a British English dictionary.
The biggest advantage for me, though, is simply that pages load much, much faster in Firefox than they do in Internet Explorer.
You ask about bad points. Well, there are a few websites which won't load in Firefox. In particular, many banks seem to have websites which are specifically designed to work only with IE.
There are also a few minor bugs which affect the way FF works with Windows Vista. (For example, it's not currently possible to set Firefox to be your default browser in Windows Vista). It's likely that these problems will be fixed before Windows Vista is widely used.
Firefox takes longer to load up when you first start it (but that's more than compensated for by the much faster times that pages take to load once it's running). Dial-up users, who've got IE configured so that Dial-Up Networking automatically connects them to their ISP at start-up, have to connect to the net first and then start FF.
Despite those few 'minus points', for me (and for about 80% of the 'regulars' here in AB's Technology section) Firefox is easily the best browser.
Chris