A way to enable flash for specific sites only?

You could use an add-on to do this.

NoScript Firefox

Flashblock Firefox

Flashblock Chrome

The procedure for the current version of Chrome is detailed below.

Open a tab with chrome://flags and verify the the following are set to Default : Prefer HTML over Flash and Run all Flash content when Flash setting is set to “allow”
Open another tab with chrome://components and under Adobe Flash Player, click Check for update and wait for it to finish
Click on the Chrome menu button at the top-right and then on Settings
Click on Show Advanced Settings
Under Privacy click on Content Settings
In the dialog, under Flash ensure that Ask first before allowing sites to run Flash (recommended) is selected (don't do Allow sites to run Flash)
Now Flash is allowed and you need to enable it for your specific sites
The hard way to specify an allowed site is under Content Settings – Flash to click on the Manage exceptions button and add each site's URL with Behavior set to Allow
The easy way to allow a site is to go to it and then click on the little icon to the left of the URL in the address bar, and set Flash to Always allow on this site instead of the default which does not work in the latest version of Chrome.

More detailed instructions with screenshots can be found in this article.

In addition to aforementioned solutions, for Firefox there is a native (built-in) one too:

Click on the icon in the beginning of the address bar in Firefox enter image description here

And now you have full control over this page.

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