After theme FontAwesome update to v.5 my plugin icons show up as square

This might be due to your plugin using a FontAwesome version older than 5.

FontAwesome v5 is not fully compatible with its previous versions, that's why you see your plugin icons showing up as square.
In most of the cases this can be solved by adding the following chunk of CSS in Appearance -> Customize -> Advanced options -> Additional CSS:

.fa {
    display: inline-block;
    font: normal normal normal 14px/1 FontAwesome;
    font-size: inherit;
    text-rendering: auto;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}