Is this a problem with pasting? Firefox does something weird with the transparent areas of PNGs when pasting. You may need to save those or use a different format. (Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=460969)

If it's a problem with saving, could you share the address of a problem image? You can contact me either at jscher2000@outlook.com or start a new issue thread on https://github.com/jscher2000/Save-webP-as-extension/issues