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About:config
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The about:config page is a way to get at some more advanced options. I only know about what a few of the settings in there will do, but I will tell you what I know. Some of the settings on that page correspond with the settings in the Tools > Options settings. If you type about:config in the address bar you will get a page like this.
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I'll start from the top and give the options that I have used that aren't in the regular options of Firefox. Like I said, compared to the number of them that are there I've altered very few.

1.browser.chrome.favicons

2.browser.chrome.site_icons these two are responsible for the little icons on bookmarks or in the address bar or tabs. I don't like different icons for each website, it bugs me. If you just want the default icons right click and press toggle.

3.browser.download.manager.showAlertOnComplete this is for when a download completes there is a little pop up by the clock that tells you it's done. If you want to stop that right click and press toggle.

4.browser.throbber.url this is for when you press the throbber in the right upper corner under the close button(x) you can change this to the page you want, I just leave mine default.

5.dom.disable_window_open_feature.location

6.dom.disable_window_open_feature.menubar

7.dom.disable_window_open_feature.minimizable

8.dom.disable_window_open_feature.personalbar

9.dom.disable_window_open_feature.resizable

10.dom.disable_window_open_feature.scrollbars

11.dom.disable_window_open_feature.status

12.dom.disable_window_open_feature.titlebar

13.dom.disable_window_open_feature.toolbar

What these are for?(5-13) You know how sometimes you click a link and it will open a new window and that window will not have any toolbars on it or you can't resize it or minimize it? This is called a javascript window and these settings all disable a website's ability to take away these options. For example sometimes I shop at bestbuy.com and when I click the weekly special link it opens in a new window with no toolbar or address bar. So I toggle #5 and #13, then when I go to the weekly specials link the window opens with the address bar and back and forward buttons on it. You take the last word in these settings and if you want to be able to do it(minimize, resize) or see it (any toolbar) all the time, then right click and toggle the setting. These are all the settings I have worked with but they may make a difference to you like they do to me. Check them out.
You can see the settings from some extensions in the about:config page. With the, over 200 to date, extensions you can do more things with Firefox than any one person can think of. The next page will be dealing with installing extensions.
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Here's is a good Firefox Help Site