If you are using Flash you need to also install nspluginwrapper, and use it to "wrap" the plugin so WebKit can see it.
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Use "yum -y install nspluginwrapper" to install the package.
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The OLPC OS build scripts for custom builds use the following command (run as root or via sudo) to finish the wrapping process. I have not verified it yet on a XO-1.5: