Adobe Flash 11.3 isn't all that stable... There's some workarounds listed on the linked Mozilla Support page.
Personally, I would make sure Flash is up to date first, and possibly uninstall it (through Windows
Programs and Features) and reinstall it (http://get.adobe.com/flashplayer/).
The next step is to try disabling hardware acceleration (right click on any Flash applet and click settings).
Finally, the workarounds on that page:
If it's crashing, install 10.3 or 11.2 (http://mzl.la/LdjZH6)
If Flash based videos and games are not loading correctly (but not actually crashing), first try turning off the web download feature of RealPlayer. Then try removing the RealPlayer Firefox extension. Finally, try uninstalling Realplayer. If none of that works or you do not have RealPlayer installed, try downgrading Flash (see above) (http://mzl.la/LA0mpi)
It seems to be a new "protected mode" feature: http://kb.mozillazine.org/Macromedia_flash#Flash_Player_11.3_Protected_Mode_-_Windows
Disabling Protected Mode in Flash 11.3
You can disable Protected Mode in Flash 11.3 by adding the line
ProtectedMode=0 to the Flash
mms.cfg file located in:
(Windows 32bit) C:\Windows\System32\Macromed\Flash
(Windows 64bit) C:\Windows\SysWOW64\Macromed\Flash
For example, on a Windows 7 64-bit system, open the
C:\Windows\SysWOW64\Macromed\Flash folder, copy the file
mms.cfg to the desktop, open it in Notepad, add a new line
ProtectedMode=0 and then close Notepad, saving your changes. Next, go back to the
C:\Windows\SysWOW64\Macromed\Flash folder, rename the original file to
mms.cfgBAK (to save it as a backup) and then drag and drop in the modified
mms.cfg file from the desktop. You can later re-enable protected mode by either restoring the backup or by editing the
mms.cfg file to remove the
ProtectedMode=0 line you added.
Full links (in case the above ones break)