Flash is a web browser plugin that is used in almost all websites. For example, whenever you view a video on the web, or play a game in browser, ≈99% of the time it is using Flash. Also, almost all online ads that shows some animation are using Flash. Even for sites that do not have any animation, they usually have invisible Flash “bug” that tracks your click behavior.
Flash cookies can track your browsing behavior, but it doesn't know your name. However, if you use social networking sites such as Twitter, Facebook, chances are high that some rogue sites also know your name, address, phone number, and these info are stored in their data center.
To delete Flash cookies, go to this page: http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager06.html
Bookmark that link. Whenever you want to delete Flash cookies, you have to go there.
You can view and delete the local Flash cookie files on your disk. They are technically called Local Shared Objects. For example, on Windows Vista and 7, it's at 〔C:\Users\yourname\AppData\Roaming\Macromedia\Flash Player\#SharedObjects\〕. For Mac OS X and Linux and other tech detail, see Local Shared Object.
Flash also has a “global” cookie, and you must go to the site mentioned above to delete info there.
The bad thing is that some sites won't let you play video or other things if you don't let them store info on your computer.
You can disable Flash in Firefox without uninstall it.
Go to menu 〖Tools ▸ Add-ons〗. Then click “Disable” button for Flash.
This way, you can use Firefox for normal browsing and use another browser such as {Google Chrome, Safari, Opera}, for sites you need Flash.