Work around Firefox's transient activation requirement for opening the file picker (allow using shift as modifier)

ui/script.js

@@ -749,8 +749,9 @@ document.body.addEventListener('keydown', e => {
     return
   }
 
-  // Ctrl keys
-  if ((e.ctrlKey || e.metaKey) && !e.shiftKey) {
+  // Modifier keys
+  if (!e.shiftKey) {
+    if (e.ctrlKey || e.metaKey) {
       switch (e.code) {
         case 'KeyC':
           return prevent(e) || isRo() || cpPath()
@@ -784,6 +785,17 @@ document.body.addEventListener('keydown', e => {
           return prevent(e) || dl(getASelected())
       }
     }
+  } else {
+      // Workaround Firefox requirement for transient activation
+      // https://developer.mozilla.org/en-US/docs/Web/Security/User_activation
+      // Firefox requires user interaction (that is not reserved by the user agent)
+      // before a file picker can be displayed. This means that ctrl/meta are not
+      // usable as modifiers until the user clicks the page or presses another
+      // non-modifier key. To work around this, the shift key can be used, instead.
+      if (e.code == 'KeyU') {
+          return prevent(e) || isRo() || manualUpload.click()
+      }
+  }
 
   switch (e.code) {
     case 'Tab':