Use weakmaps again now that bug 819131 was fixed in Firefox 20.

This also silences a warning for getUserData and setUserData added in bug 881252.

I notice this all of the time and it was reported on the forums at https://adblockplus.org/forum/viewtopic.php?f=11&t=15338&p=76393 and https://adblockplus.org/forum/viewtopic.php?f=11&t=15732&p=77212 (I don't have a forum account to point here).

The test suite seemed to have the same number of failures before and after this change. I didn't do manual testing because I didn't look at what the weak maps are used for. I mostly want to silence the warning.

This is a partial revert of 987f2b9.