Firefox is Really Slow

Firefox is extremely slow on my system, include menu, right-click, tab opening…etc.
Mozilla Firefox 40.0
Xubuntu 14.04 LTS 64-bit
Linux 3.16.0-45-generic #60~14.04.1-Ubuntu SMP Fri Jul 24 21:16:23 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

After I turned off hardware acceleration, everything is fine.
Type this in the browser’s address bar to trun it off:

about:preferences#advanced