Thanks for the help Elotro, since my main browser is Firefox that disappearing icon thing was driving me insane, however I fixed it with a really easy edit.
filter: url('inc/desaturate.svg#greyscale'); /* Firefox */
is a reference to the file desaturate.svg which is contained in the inc directory which is inside of the spun theme directory.
Since we have been using child themes, this path was no longer valid because we did not copy the inc directory into the child theme’s directory.
So this can be easily fixed either by copying the inc directory into the child theme, or by changing your path name in the code to reference the correct location, like so:
filter: url('../spun/inc/desaturate.svg#greyscale'); /* Firefox */
Hope this helps!