Confirmed that disabling USB sharing workaround has fixed the issue for me on a Fedora 19 3.11.10-200.fc19.x86_64 box. No kworker taking CPU cycles:
2452 fidel 20 0 3020952 0,978g 35172 R 60,6 12,5 647:20.88 /usr/lib64/firefox/firefox -new-instance -P fidel
2567 fidel 20 0 2008472 737540 26644 R 39,0 9,0 467:10.20 /usr/lib64/firefox/firefox -new-instance -P TX
1060 root 20 0 181556 49356 31632 S 17,2 0,6 321:12.98 /usr/bin/X -background none :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt1 -novtsw+
2818 fidel 20 0 688040 93732 11504 R 11,9 1,1 102:27.61 /usr/lib64/firefox/xulrunner/plugin-container /usr/lib64/flash-plugin/libflashplayer.so+
2587 fidel 20 0 1948312 654592 26764 S 9,3 8,0 390:07.07 /usr/lib64/firefox/firefox -new-instance -P JL
4005 fidel 0 -20 1523064 113364 41048 S 7,9 1,4 0:18.45 /usr/NX/bin/nxnode.bin
2905 fidel 20 0 688040 102500 11516 S 2,6 1,3 69:42.64 /usr/lib64/firefox/xulrunner/plugin-container /usr/lib64/flash-plugin/libflashplayer.so+
4402 fidel 0 -20 1374364 70956 11248 S 0,3 0,9 0:01.21 /usr/NX/bin/nxnode.bin -H 6
4557 root 20 0 123696 1672 1160 R 0,3 0,0 0:00.07 top
32548 root 20 0 0 0 0 S 0,3 0,0 0:01.28 [kworker/u8:0]