nightly crashes when viewing HTML5 video

[AN]

FreeBSD FBSD10 10.0-CURRENT FreeBSD 10.0-CURRENT #23 r237852: Sat Jun 30 
18:45:27 EDT 2012     root at FBSD10:/usr/obj/usr/src/sys/MYKERNEL  amd64

# svn up
Updating '.':
At revision 871.

I have been using FF nightly for several weeks and it had been working 
well with HTML5 video.  I started having a problem today watching HTML5 
videos, it causes the broswer to crash immediately.

I loaded sem.ko but the crash still occurs:

]# kldstat
Id Refs Address            Size     Name
  1   22 0xffffffff80200000 d0d840   kernel
  2    1 0xffffffff80f0e000 10ae520  nvidia.ko
  3    3 0xffffffff81fbd000 48450    linux.ko
  4    1 0xffffffff82006000 4ef30    vboxdrv.ko
  5    1 0xffffffff82212000 3df6     linprocfs.ko
  6    1 0xffffffff82383000 a9f3     fuse.ko
  7    1 0xffffffff8238e000 23d4     sem.ko



Also, another problem appeared after the crashes started, tabs are on top 
with no way to move them to original position.  Under the menu item "view" 
-> "toolbars" the Tabs on Top option is missing.

I have verified both of these are working fine in FF 13.  Is anyone else 
seeing these problems?  What data can I try to collect to help 
troubleshoot the problems?

Thanks in advance.