- Right-click on Firefox / Chrome in your taskbar
- Right-click on Firefox / Chrome in the list, and select “Properties”
- “Compatibility” -> “Change settings for all users”
- “Run this program” as “Windows 8” in the drop-down list.
- OK until you get out of the menus. Restart Firefox/Chrome as needed.
The WebEx plugins apparently test for what OS they're running on. The version string difference by the 8.1 upgrade causes it to declare as unsupported, even though 8.1 didn't really modify anything WebEx would use. I assume a future update will fix this.
Added: using compatibility mode might be breaking Adobe Flash. Use compatibility mode only as needed.