Came across a bug in Firefox 25.0.1 that affects add_more fields.
Unable to focus/select/edit inputs in add_more fields when clicking on them (tabbing to the field still works).
This post on stackoverflow led me to a solution:
http://stackoverflow.com/questions/9804026/clicking-on-a-textbox-in-firefox-doesnt-give-focus
Bug is caused by .disableSelection() on line 379 of piklist/parts/js/piklist.js
I simply commented out that line and everything works fine now.