Update firefox version to 75 (#12000)

uitest/src/test/java/com/vaadin/tests/VerifyBrowserVersionTest.java

                     getDesiredCapabilities()) + "81";
         } else if (BrowserUtil.isFirefox(getDesiredCapabilities())) {
             browserIdentifier = getExpectedUserAgentString(
-                    getDesiredCapabilities()) + "58";
+                    getDesiredCapabilities()) + "75";
         } else {
             browserIdentifier = getExpectedUserAgentString(desiredCapabilities)
                     + desiredCapabilities.getVersion();