Fix bug where signature couldn't be added above the quote in Firefox 51 (#5628)

CHANGELOG

@@ -9,6 +9,7 @@ CHANGELOG Roundcube Webmail
 - Fix PHP error on update of a contact with multiple email addresses when using PHP 7.1 (#5587)
 - Fix bug where mail content frame couldn't be reset in some corner cases (#5608)
 - Fix bug where some classic skin images were not displayed in IE/Edge (#5614)
+- Fix bug where signature couldn't be added above the quote in Firefox 51 (#5628)
 
 RELEASE 1.3-beta
 ----------------

program/js/app.js

@@ -4212,6 +4212,8 @@ function rcube_webmail()
       // add signature according to selected identity
       // if we have HTML editor, signature is added in a callback
       if (input_from.prop('type') == 'select-one') {
+        // for some reason the caret initially is not at pos=0 in Firefox 51 (#5628)
+        this.set_caret_pos(input_message, 0);
         this.change_identity(input_from[0]);
       }