Based on Leechblock by James Anderson:
http://www.proginosko.com/leechblock.html

Here's a quick little hack I put together, which adds a warning on any non-whitelisted site.
I also added a context menu for whitelisting or blacklisting.

It's really a quick and ugly hack, so the code can probably be improved a lot.
The allow, ban and leave buttons on the block page currently also don't work yet.

It's based on Leechblock 0.5.2.

XPI creation:
Type "make" to create the .xpi (on Windows create a zipfile named leechblock_custom.xpi containing: chrome  chrome.manifest  defaults  history.txt  install.rdf)

Installation:
Open the .xpi file with firefox

License: GPLv3