Menu does not display in Firefox 13 on Xubuntu 12.04

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http://askubuntu.com – I had a problem after update firefox to version 13. When I start the firefox, it works. But for a while I can't click on firefox menu anymore. The right click menu doesn't work either. I'm using Xubuntu 12.04 64 bits: Linux minh-desktop 3.2.0-26-generic #41-Ubuntu SMP Thu Jun 14 17:49:24 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux. Everytime I get this problem, I have to restart the firefox. Please help. Thank you. (HowTos)