Update IS_FIREFOX_LEGACY flag to prevent FF crash (ianstormtaylor#4237)

* Update IS_FIREFOX_LEGACY flag to prevent FF crash

ianstormtaylor#4118 and ianstormtaylor#4150 introduced IS_FIREFOX_LEGACY, but this block of code was not updated to reflect that. This breaks insertion of emoji and unicode characters in slate-react. This change fixes ianstormtaylor#3855.

* Update editable.tsx

* Create forty-mails-roll.md

* Update editable.tsx

Apologies, in my tests I was using yarn patch and didn't need to modify the import statement, hopefully this will pass CI now.

Co-authored-by: Ian Storm Taylor <ian@ianstormtaylor.com>
# Conflicts:
#	packages/slate-react/src/components/editable.tsx

packages/slate-react/src/components/editable.tsx

@@ -18,6 +18,7 @@ import Hotkeys from '../utils/hotkeys'
 import {
   IS_IOS,
   IS_FIREFOX,
+  IS_FIREFOX_LEGACY,
   IS_SAFARI,
   IS_EDGE_LEGACY,
   IS_CHROME_LEGACY,
@@ -634,7 +635,7 @@ export const Editable = (props: EditableProps) => {
               // aren't correct and never fire the "insertFromComposition"
               // type that we need. So instead, insert whenever a composition
               // ends since it will already have been committed to the DOM.
-              if (!IS_SAFARI && !IS_FIREFOX && !IS_IOS && event.data) {
+              if (!IS_SAFARI && !IS_FIREFOX_LEGACY && event.data) {
                 Editor.insertText(editor, event.data)
               }
             }