Mac and Linux Preferences

On a Mac and Linux you need to set two Firefox preferences.

You access preferences by typing about:config in the window where http://... usually goes.

Type instantapply in the Filter search box. This should bring up a list of preferences. Look for browser.preferences.instantApply.

When you find it, do a double click (right click if your mouse has more than one button?) on the Value (4th. column) "true" to change it to "false".
Alternatively, do a Control-Click and select Toggle. This should also change the Value to "false".
If you do not make this change, you won't see the OK and Cancel buttons, which stops you from setting any plugin preferences.
Search for that instantApply in this article for more info about these preferences (in the article's Contents box, click on #6. Browser).

Next, look for browser.preferences.animateFadeIn in the same list. If you don't see it, type animatefadein in the Filter search box.

When you find it, do a double click (right click if your mouse has more than one button?) on the Value (4th. column) "true" to change it to "false".
Alternatively, do a Control-Click and select Toggle. This should also change the Value to "false".
If you do not make this change, some plugin popup windows may be blank.