The process for validating Firefox is almost identical to that for Google Chrome. Select Tools | Web Developer | Toggle Tools from the menu bar or use the keyboard shortcut Cmd/Ctrl + Shift + I:

Accessing Developer Tools in Mozilla Firefox
Select the Console tab, click inside the command input box, type WebAssembly, and press Enter. You'll see this if your version of Firefox is valid:

Results of WebAssembly validation in Mozilla Firefox's Developer Tools console