Embedded Flash is Blocked in Firefox

Have you ever installed Adblock plugin to your Firefox? Right, in the plugin preferences, you can set the browser to block the ads and embedded object. But now, the same effect, occurs in my without-Adblock-Firefox. When I open a site that contains an embedded Flash object, a strange gray box hide it, so I can’t interact with the object. First I thought that the Flash Player plugins didn’t work, but the browser didn’t show a notification about the ‘Missing plugin’. I get confused why does it happen? I never installed adblock before. Plugins installed in my Firefox only Firebug and Web Developer, for work purpose.

After I cleared all cookies and the private data, the flash player work well again. One thing that I realized is, the gray box will appears again, after I open a file sharing site, http://indowebster.com. Indeed, in that site there are a lot of ads and Flash object. After those objects loaded, pooff!!, the gray box suddenly appears. Not only in that site tab, but in every tab in my Firefox.

Until now, I didn’t get that problem solved, even after I re-installed the browser. Maybe anyone can help?
Thanks in advance