I've not used a Mac OS, was it working ok up till recently with Firefox? do you think there's a possibility there was an update to the OS that changed something? idk if you can reinstall graphics drivers?
idk if you're aware, in Firefox menu, options, performance section, if you un-tick the box for "use recommended performance settings" a new option appears "use hardware acceleration when available" tick that one and see if it's any different.
Firefox uses the NPAPI version of the flash player plug-in, in Firefox menu, add-ons, plug-ins, shockwave flash should be set to "always activate"
idk about Mac OS but in Windows you can go to Control Panel and open the flash player applet, under "local storage settings by site" you should have the current cookie for the game "playerio-a.akamaihd.net"
if this cookie is absent you need to go to Firefox Menu, options, privacy & security, Cookies & Site Data, Accept cookies and site data from websites (recommended)
you need the third party cookies option too for Facebook but it shouldn't be needed for Armor.
otherwise, as a workaround to allow you to play, you could download and install the Yandex browser, it's based on the chromium project like Chrome, Opera & Vivaldi but doesn't suffer from the software rendering problem that makes Chrome useless now.
I hope you manage to overcome your current hitch!