toolbar_button.js should do more null checks
I have installed it on WinXP, Win7, Linux(Ubuntu) in Firefox 29.0.1. On XP, I see a long list of rules in the preferences screen. On Win7 and Linux, the rule list is empty. I have tried reinstalling and restarting, but to no avail. I can't tell if it's actually doing anything or not without any rulesets...