Method to produce this skeleton Firefox profile directory:

Create an empty profile directory.
Unpack the XPI into $PROFILE_DIRECTORY/extensions/https-everywhere@eff.org.
This "installs" the extension but doesn't really install it. At first start of
Firefox, you will be prompted whether you want to install the extension.

Run firefox -profile $PROFILE_DIRECTORY, but *do not* click install yet.
Instead, cp -r $PROFILE_DIRECTORY to $PROFILE_DIRECTORY.copy. Then go back to
Firefox and click install. Quit Firefox, and run diff -r $PROFILE_DIRECTORY,
$PROFILE_DIRECTORY.copy. The files that differ are the ones that are required
for Firefox to consider the extension installed. Use some judgement to determine
which ones are critical, modify them as necessary (for instance, enable verbose
logging in prefs.js), and save them to this skeleton directory.