UPDATE:
1 out of 5 stars, because this is now spyware.

In chrome\blocksite.jar!content\BlockSitePrefs.js on line 53https://addons.mozilla.org/en-US/firefox/files/browse/83818/file/chrome/blocksite.jar/content/BlockSitePrefs.js#L53locationArray.push(locationList.getItemAtIndex(i).label);The function locationList.getItemAtIndex() can return undefined if the index is not visibly displayed, this causes BlockSite to generate regular expressions that flag any web address as blocked.In other words, once the blacklist gets too long and doesn't fit in the dialog BlockSite will start blocking everything. Or you start to lose some of the blacklist.The fix for this bug is to put in this line before 53:locationList.ensureIndexIsVisible(i);This ensures the item is visibly displayed thus stops the above bug glitch from happening.The file BlockSitePrefs.js can be found in the xpi file {dd3d7613-0246-469d-bc65-2a3cc1668adc}.xpixpi and jar files can be extracted as a zip filewindows:%AppData%\Roaming\Mozilla\Firefox\Profiles\(profile id).default\extensionsmac:~/Library/Application\ Support/Firefox/Profiles/(profile id).default/extensions