When using Firefox and encounters "406 Not Acceptable: An appropriate representation of the requested resource / could not be found on this server" error, make sure all private data are cleared and restart Firefox to resolve the issue.
If you simply tried to uninstall Firefox and reinstall, it will not resolve the issue.
If you enabled the option to "Always clear my private data when I close Firefox" and restart, it will not resolve the issue. This may be some indication that clear private data on exit option failed to do a proper clean up job.
To resolve the problem, click Tools->Options. On the Privacy tab, check "Ask me before clearing private data" then click Clear Now. On the Privacy tab, check "Ask me before clearing private data" and click Clear Now. At Clear Private Data dialog box, select everything then click Clear Private Data Now. This will resolve the problem.