By doing so, you will remove cache for all apps.
You can focus on one particular app by doing something like:
```
var cacheService = Cc["@mozilla.org/network/application-cache-service;1"].
                            getService(Ci.nsIApplicationCacheService);
cacheService.discardByAppId(appId, false);
```

See this IDL for more info:
http://mxr.mozilla.org/mozilla-central/source/netwerk/base/public/nsIApplicationCacheService.idl#72