Many Firefox settings can only be changed in the about:config dialog that appears when you load the command in the Firefox address bar. The settings in there however have rather cryptic names and it can sometimes be quite hard to figure out what they do. You need to research the parameter on the Internet, and most are explained on Mozilla's own website. But if you do not know that, you may spend lots of time researching parameters on the Internet.
Configuration Mania is a Firefox add-on that displays many of those "advanced" settings in a menu that resembles the Firefox options menu. Users can access five main tabs, like Browser or Security, which lead to several subtabs that provide access to the settings.
Let me give you an example that helped me actually. Clicking on the Browser tab opens a menu with several like User Agent, Location Bar and Download. A click on Download revealed the option to disable the alert that you receive when a download finishes. I would have a hard time finding that option in the about:config settings.
Users who would like to keep the alert can chance the interval in that it is shown and even increase or decrease the speed of the animation. You can find dozens of such helpful settings in Configuration Mania.
Update: Config Descriptions is an extension for Firefox that adds so called source comments to preferences in the about:config dialog. While not available for all parameters, it does add descriptions to many of them, especially those that are standard preferences.
You will however notice that extension preferences, parameters added by extension installations, are not explained here at all. Still, it is a great option to add information to Firefox's about:config dialog.
There is also the What's that preference add-on which you can use to open the MozillaZine article directly, if available.
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