Seems the problem is caused by too many sandboxing methods stacked. Flash introduced "protected mode" in its plugin which causes no end to trouble for Gecko browsers that are already using OOPP.
Adobe integrated Protected Mode for Firefox in this release, and it appears that it is this new feature that is causing the issues for Pale Moon users. If you are interested in background information on Protected Mode for Firefox, I suggest you check out the official announcement of it over at the Adobe website.
Adobe itself recommends disabling Protected Mode until a fix comes out that is resolving the issue.
Our Protected Mode implementation allows Flash Player to run as a low integrity process with several additional restrictions that prohibit the runtime from accessing sensitive resources.
By running the Flash Player as a restricted process, Adobe is making it more difficult for an attacker to turn a simple bug into a working exploit.
To disable Protected Mode you need to edit the mms.cfg file located in one of the two directories:
Windows 32bit: C:\windows\system32\macromed\flash
Windows 64bit: C:\windows\syswow64\macromed\flash
Just open the file with a text editor (you may have to run it as Administrator) and add the following line to the mms.cfg
ProtectedMode = 0
If the file does not exist, create it first.
To enable Protected Mode again simply delete the line that you have added to the config file.