About .XPI File Format
To do word count in XPI archives, let’s get to know a little more about this format.
XPInstall (Cross-Platform Install) is a technology used by SeaMonkey, Mozilla Firefox, Mozilla Thunderbird and other Mozilla software for installing extensions to the main program.
An XPI (pronounced “zippy”) module is a ZIP file that contains an install script or a manifest at its root, and a number of data files.
Since Firefox 1.0, XPIs from sites other than Mozilla Add-ons are blocked by default, to fight malware. Third party modules can be whitelisted, but malicious sites cannot install new extensions without the user’s intervention.
Since it is based on the ZIP format, many common software utilities can create, modify or open XPI files, notably 7-Zip, WinZip and WinRar.
Link:
WinZip Home Page
7-Zip Home Page
WinRAR Home Page