Firefox lets you disable plug-ins if you believe they are causing your browser to crash or are using too many resources on your PC. For this reason, it's possible that you've disabled the Flash plug-in for your browser. Since you need this plug-in to watch the majority of online videos and to view some websites, having Flash enabled ensures you're not missing anything. If you've disabled Flash, it takes just a few steps to re-enable it.
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Start Firefox, then click the "Tools" menu button in the menu bar and select "Add-ons."
Open the "Plugins" tab.
Scroll down to the entry called "Shockwave Flash" and click the "Enable" button to the right of the plug-in's name. Flash for Firefox is now re-enabled.