This could be due to a couple of things....
Has your Firefox been updated to the most recent version? If it has, we ask that you follow the instructions in the following link
http://forums.myvmk.com/threads/a-fix-for-firefox-52.66655/
The newest version of Firefox no longer supports Shockwave.
You could also try clearing your cookies/cache in your browser to see if that gets things running smoothly again.
If all else fails, you can always download myVMK Pal.
Please let us know if the following solutions do not work.
Thanks!
-Kida