From MozillaZine Knowledge Base
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Windows users have two choices for playing RealMedia: the official Real Player or the ad-free Real Alternative. Both will work in Firefox and SeaMonkey, but Real Alternative version 1.32 or later has the advantage of including a "modified plugin" that overcomes a problem with incorrect coding on certain webpages containing embedded Real media, as explained here.
After the Player installation is complete, start Firefox/SeaMonkey and enter about:plugins into the Category:Location Bar. "RealPlayer(tm) G2 LiveConnect-Enabled Plug-In (32-bit)" and should be listed.
RealPlayer Browser Record Plugin extension
RealPlayer includes a Web Download & Recording feature that adds a Browser Record Plugin add-on, which despite the name is actually an extension, not a plugin, and appears in the Firefox or SeaMonkey Add-ons Manager Extensions list. This extension has been known to cause issues with Flash content or other problems in Mozilla browsers (see the Problematic extensions article). You can disable the extension in the Add-ons Manager or you can remove it by turning off the Web Download & Recording feature in RealPlayer (open RealPlayer, go to "Preferences -> Download & Recording" and uncheck "Enable Web Download & Recording for these installed browsers", then click OK, close RealPlayer). If you turn off Web Download & Recording in RealPlayer preferences, the RealPlayer Browser Record Plugin will be removed from the Add-ons Manager Extensions list when you next start Firefox or SeaMonkey.  
Flash Protected Mode issues
Starting in Flash Player 11.3, the Flash plugin used in current versions of Firefox and SeaMonkey runs in a Protected Mode on Windows Vista and above. Conflicts with RealPlayer's Web Download & Recording feature and this new security feature can cause An error occurred. Please try again later messages at YouTube, videos on other sites may fail to display or you may have other problems with Flash content. Because of these issues, the RealPlayer Browser Record Plugin extension was added to the Add-ons Blocklist on June 12-14, 2012 for versions 15.0.4 and below.   Version 15.0.5 was blocklisted on July 10, 2012.   Updating to RealPlayer 15.0.6 (or above) should resolve these issues and allow you to continue using the RealPlayer Browser Record Plugin extension with Firefox or SeaMonkey. Alternately, you can use following workaround:
You can then turn on "Web Download & Recording" in RealPlayer preferences (if you turned it off) and you can re-enable the RealPlayer Browser Record Plugin extension from the Add-ons Manager Extensions list (if it's disabled).
Windows plugin issues
In recent Mozilla browsers (e.g., Firefox 4 and above) the RealPlayer G2 LiveConnect-Enabled Plug-In (file name: "nppl3260.dll") will crash on sites with embedded Real Media such as the Test your Real media player page at www.slp3d2.com linked from Testing plugins (see bugs 667399 and 460864). You can disable the "RealPlayer(tm) G2 LiveConnect-Enabled Plug-In" in the Add-ons manager Plugins list to avoid such crashes. Note: Direct links to Real Media files (such as this .ram file can still be opened in the RealPlayer application itself, even though the plugin is disabled.