When a Firefox for Android user submits my website subscription form without filling in the required fields, there is no apparent response. An alert reading "please fill out this field" positioned near the first required input field should be displayed. Other browsers on the same mobile device display the alert correctly. Firefox (and other browsers) on Windows display the alert correctly. To reproduce the error, 1) visit https://huppbrian.us 2) open the navigation menu and select subscribe 3) submit the popup subscription form without entering anything into the e-mail field