The Flash Control add-on for Firefox will let you control over which sites you choose to allow Flash Player to display. It’s designed with YouTube in mind.
Here are more details about the add-on from the developer:
The icon is NOT backwards. RED means you’re “vulnerable” to Flash; GREEN means you’re safe, ie you don’t have to worry about Flash meddling with your browser. RED is also the color used by Adobe Flash, so when you see the “Adobe Flash” icon, it means it’s active. There’s nothing to fix in that regards, it’s part of the design.
There are some limitations to the current add-on api, and it takes a little bit of planning (aka time) to implement a whiltelist. Or, I could port the code back to the “old api”, which is more robust and versatile, but I would again need time to re-write and test out the code.
Learn more about Flash Control at the Firefox Add-ons website.