It's simpler to just get rid of it altogether. You can always open blocked pop-ups from the icon on the right side of the address bar.

1. Type about:config into the address bar and press Enter.
2. To bypass the warning, press the big button labeled "I'll be careful, I promise!".
3. In the search box, paste privacy.popups.showBrowserMessage
4. In the search results, double-click privacy.popups.showBrowserMessage to set its value to false.

Thank Gingerbread Man, this solves half the problem!

Is there a way to do the same for messages about allowing flash, for example?

Are you referring to the missing plug-in notification for Flash Player pictured below? If so,

1. Type about:config into the address bar and press Enter.
2. To bypass the warning, press the big button labeled "I'll be careful, I promise!".
3. In the search box, paste plugins.notifyMissingFlash
4. In the search results, double-click plugins.notifyMissingFlash to set its value to false.
   • The icon will still appear on the left side of the address bar, but the notification won't pop up unless you click it.

Firefox 30 (currently in the beta channel) also has the following preference:

• plugins.hideMissingPluginsNotification

If you mean something else, please attach a screenshot. A link to the page would likely also help.

• How do I create a screenshot of my problem?

You're welcome.

You can get a possibly colored Lego block icon on the left end of the location bar that indicates that Firefox is blocking a plugin.

• https://support.mozilla.org/kb/why-do-i-have-click-activate-plugins
• http://kb.mozillazine.org/Issues_related_to_plugins#Click_to_Play