From MozillaZine Knowledge Base
By default, extensions are installed for the current user only, but they can also be installed across multiple profiles and even globally.
Once installed, extensions can be configured by selecting "Extensions" from the the Add-ons Manager ("Tools -> Add-ons -> Extensions") selecting the extension from the list, and then clicking the "Options" button. (If this button is disabled then the extension is not user-configurable.)
If you experience problems installing or updating an extension read this article for a list of possible causes and solutions before visiting the author’s web page for that extension to check for known issues.
Extensions installed in this way will not appear in any other profiles which currently exist or which are later created.
Multi-user installations are useful if more than one person uses your application. You should install and use the MR Tech's Toolkit extension to configure your extension installations for multiple users. Alternatively you could simply allow multiple users to use the same profile; you must ensure that the profile folder is accessible to each of them on your system.
A global installation will install an extension to the application directory rather than within a profile, so it will be available to all users. To perform a global installation you should not activate the .xpi installer file within your Mozilla application. Instead, download and save it to disk and ensure that you close the application completely.
Then follow one of the following options:
where "extname.xpi" is the name of the installer file. This command line option has been dropped as of Gecko 1.9.2.
Depending on the extension, each user may need to configure a globally-installed extension independently.
If you want to do a silent administrative install that immediately makes the extension available to limited users, you need either:
Changing installation status
If you wish to change the installation type of an extension from single-user to global (or vice versa), or move or copy it from one user profile to another, you should uninstall the extension and perform one of the installation methods above.
Advanced users may wish to try moving the extension files manually between the appropriate locations, as described below. Note that although this procedure should work on version 1.5 and above of Firefox and Thunderbird, and perhaps on versions 1.0.x, it is not guaranteed, and it could break your extension, your profile or even your application.
Location of extension files
Extension files are located in the "extensions" subfolder of the profile folder (single-user) and in the "extensions" subfolder of the installation directory (global); there is one folder per extension, named using their ID codes. To tell which is which, look inside each folder’s "chrome" subfolder where you will find some human-readable information regarding the extension.
Note: Starting in Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1), XPI files are no longer unpacked when extensions are installed. Instead, the XPI itself is placed in the extensions directory, and files are loaded directly out of the package. See Updating extensions for Firefox 4 for details. 
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