I believe the latest version of Firefox is 53, but either way 52 no longer supports the Shockwave plug-in.
There is three things you can try.
First, you can down grade your Firefox to level 51 or below. (Which we do not recommend due to you not being able.to receive security updates etc.)
Second, you could try our lovely adminstrator's method to allow Firefox to work properly with the client. This link
http://forums.myvmk.com/threads/a-fix-for-firefox-52.66655/ should inform you of everything you need to know regarding Firefox.
Third, you could try reinstalling everything needed to run the client once you have a previous version of Firefox installed. (Shockwave, myVMK client, etc) sometimes something happens internally and it just needs to be reinstalled.
If this does not help please email us so we can look into this further.