Update session store path for newer FF versions

Seems like the location of the sessionstore.js file changed in recent versions of QS.

This is backwards compatible with older versions of FF.

@HenningJ may be interested to see this, as he wrote all the code. Basically just done a test for two different places where the session store might be saved. Its format is unchanged, which is nice :)

Fixes quicksilver/quicksilver#1993