avoid CSP issues on Firefox

lib/single-file/single-file-core.js

@@ -1300,7 +1300,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 								if (Util.testValidURL(resourceURL, baseURI, options.url)) {
 									let { content } = await batchRequest.addURL(resourceURL, true);
 									if (content != EMPTY_DATA_URI && !content.startsWith(PREFIX_DATA_URI_VND) && !content.startsWith(PREFIX_DATA_URI_IMAGE_SVG)) {
-										const validResource = await DocUtil.validFont("url(" + content + ")");
+										const validResource = await DocUtil.validFont(urlFunction);
 										if (!validResource) {
 											content = EMPTY_DATA_URI;
 										}