Editing Configuration files
Mozilla is an extremely customizable program, who's possibilities may be a little overwhelming at first. Many settings can be edited by using the Edit menu and clicking on Preferences, or Mail & Newsgroups Account Settings. There are many more settings that can be edited; and this page will show you how to do it.
The simplest way to edit hidden preferences is to type about:config in the browser location bar, and press enter (or click on GO).
This will display a list of used preferences, as well as a search bar on top.
When a preference is set to anything other than the default setting, it will be displayed in bold.
If you want to edit a preference setting, open the context menu for the preference (Usually by right-clicking on it), and select Modify.
You will then get a text field, in which you can change the value of the setting.
Click on OK, when you are finished.
You may also add a preference setting (provided the pref name can be read).
To do add a new preference, open the context menu of any existing preference, and go to the "New" menu.
From there, you are required to know what type of preference will be added.
Once you've selected the type of preference, you'll be asked to enter the name of the preference, then you'll be asked to enter the value.
Some preferences may require that you restart Mozilla in order to take effect.
prefs.js or user.js
When a user changes a preference from the default setting, that setting is stored in the user's profile folder. The name of that file is prefs.js. You can change preferences by editing your profile's prefs.js file in a text editor, such as Notepad. Before you make any changes to this file, make sure that Mozilla is not running. You should also make a back up copy of the file.
When you view the contents of the file, you'll see that the syntax is pretty simple.
Each preference is it's own line.
The user.js file is a file you must create. It works the same way as the prefs.js, but it is 100% user controlled and takes precedence over the prefs.js. When Mozilla is started, all settings within the user.js will be copied over to the prefs.js. If the same preference name exists in the prefs.js, it will be replaced by the one in the user.js. Mozilla, on the other hand, does not have the right to manipulate user.js. That's why changes to user.js can be made while Mozilla is running. One of the pros about the user.js is that it can act as documentation of what preferences you've set manually.
To create a user.js file for your profile, open your prefs.js (make sure you have a back up copy), erase the contents, and save it as user.js (ie. replace prefs with user).
userChrome.css and userContent.css
The userChrome.css and userContent.css, like the user.js, are also user created. Much of the user interface and the way it is displayed is written in cascading style scripts (CSS), so you can manipulate it, if given the correct tags. The difference between userChrome.css and userContent.css is that userChrome.css controls elements of the user interface (see About Chrome), and the userContent.css controls elements of the actual web content. When you create these files, you must save them in the \chrome\ directory of your profile folder. By default, there are example files, called userChrome-example.css and userContent-example.css, in your profile's \chrome\ folder, that you can look at. To create them, simply use any text editor (such as Notepad), and save them with the appropriate name (userContent.css or userChrome.css). If you'd like to add more scripts, add them to the existing css files. In other words, you can only have one userContent.css and one userChrome.css. You can edit them while Mozilla is running.