How to disable Flash plugin on Firefox, but with a whitelist? I mean *really* disable it

Flashblock is what I use. Works quietly, allows whitelist.

NoScript, a Firefox addon; it lets you disable scripts, Flash and other plugins and has a whitelist.

Uninstall the Flash Player, and install Google Chrome (it includes the Flash Player plugin built-in). Browse normally with Firefox, and when you absolutely need Flash, switch to Chrome.

John Gruber explains this setup. You can even define a keyboard shortcut to open the current page in Chrome.

To make this process automated (and use Internet Explorer instead), you can use the Firefox add-on IE Tab 2. It can display a Firefox tab using Internet Explorer automatically, based on filters that you specify.