Firefox 24 ESR should be officially released today which means Firefox 17 ESR users will be automatically upgraded to Firefox 24 ESR. I want to take this opportunity to remind everyone of a major change that happened in Firefox 21 that will impact everyone upgrading to Firefox 24 ESR.

The location of a number of important files that are used to customize Firefox has changed. Here’s the list:

• defaults/preferences -> browser/defaults/preferences
• defaults/profile -> browser/defaults/profile
• extensions -> browser/extensions
• searchplugins -> browser/searchplugins
• plugins -> browser/plugins
• override.ini -> browser/override.ini

If you find that anything you’ve customized is not working anymore, these changes are probably the reason.

http://mike.kaply.com/2013/12/10/firefox-17-esr-eol-today/