Quote from ::Momzillazine:: - Installing Extensions
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.
On Windows 7, open folder: "%APPDATA%\Mozilla\"
Go to Mozilla > Firefox > Profiles > *.default > extensions
Here you'll find some xpi files, and your firefox extension .xpi will be among that list.