1. Type about:config into the Firefox location bar.
2. Right-click on the page and select New and then Integer.
3. Name it privacy.popups.disable_from_plugins
4. Set the value to 2.
The possible values are:
* 0: Allow all popups from plugins.
* 1: Allow popups, but limit them to dom.popup_maximum.
* 2: Block popups from plugins.
* 3: Block popups from plugins, even on whitelisted sites.
Just thought I'd post this in case anyone has a similar problem.