exclude FF from COLR font check

src/utils/FontManager.js

@@ -28,6 +28,12 @@ async function isColrFontSupported() {
         return colrFontSupported;
     }
 
+    // Firefox has supported COLR fonts since version 26
+    // but doesn't support the check below with content blocking enabled.
+    if (navigator.userAgent.includes("Firefox")) {
+        return true;
+    }
+
     try {
         const canvas = document.createElement('canvas');
         const context = canvas.getContext('2d');