Starting relatively recently, many computers running Firefox (V29 and 28, maybe even 27), have been experiencing problems with Shockwave: plug-in crashing, plug-in and Firefox hanging while trying to play Flash videos, videos taking forever to load, etc. I tried the usual fixes unsuccessfully. It appears that there are a number of potential underlying problems, which may exist alone or in combination. After some research, I cobbled together the following fix. This set of fixes worked for me.
Set Firefox to compatibility mode (right-click the firefox.exe icon to access compatibility mode).
In Firefox, disable use of hardware acceleration (Tools/Options/Advanced/General Tab: un-check Use hardware acceleration when available).
Perform a "nuclear cleansing" of all existing Adobe Flash-related programs
Use Control Panel/Uninstall a Program to remove all versions and components of Adobe Flash Player ActiveX, Adobe Flash Player Plugin, Adobe Shockwave Player, Adobe Shockwave for Director.