https://github.com/fred-wang/webextension-native-mathml/pull/15 (merged and released)
After you close the math editor, you cannot see the math formula you have create so in editing mode you do not see that there is a math code there (but you can see it is if you change to the code view).
This makes editing it afterwars quite dificult.
Do you happen to use the Native MathML Firefox extension? VisualEditor has become incompatible with it a few months(?) ago. Probably VisualEditor needs to be fixed, but as a workaround you can disable the extension while editing formulæ in VE.
Thank you very much for your answer. I disabled the extension and everything works again.
@Amglez: The fix has been released, so now you can re-enable the extension and it will work correctly while editing in VisualEditor.
