From this link:
- Type about:config into the Firefox location bar.
- Right-click on the page and select New and then Integer.
- Name it privacy.popups.disable_from_plugins
- 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.
Yes, Firefox does have a built-in pop-up blocker turned on by default, but it doesn't block popups created using Macromedia Flash (which has a plugin for Firefox and other browsers), like the one on Mugglenet. Using this trick, unwanted (or if you used a value of 3, absolutely all) popups will become a thing of the past...at least, until popup advertizers find another workaround.