fix: disable :focus-visible on Firefox (#2003)

* fix: disable :focus-visible on Firefox

* fix: use unambiguous closure

src/routes/_utils/supportsFocusVisible.js

@@ -1,4 +1,7 @@
 import { thunk } from './thunk'
 import { supportsSelector } from './supportsSelector'
+import { isFirefox } from './userAgent/isFirefox'
 
-export const supportsFocusVisible = thunk(() => supportsSelector(':focus-visible'))
+// TODO: remove the Firefox check once this bug is fixed
+// https://bugzilla.mozilla.org/show_bug.cgi?id=1699154
+export const supportsFocusVisible = thunk(() => (!isFirefox() && supportsSelector(':focus-visible')))

src/routes/_utils/userAgent/isFirefox.js

@@ -0,0 +1,3 @@
+export function isFirefox () {
+  return typeof InstallTrigger !== 'undefined' // https://stackoverflow.com/a/9851769/680742
+}