Mozilla release the next scheduled update of Firefox with Firefox 42 on Tuesday, November 3rd. There are several changes and updates to this version including changes to private browsing. More information is available in the Release Notes. Depending on their update settings, users should be prompted to update within 48-hours of this release or can do so in Firefox via Help > About Firefox or download and manually install the new version via the getfirefox.com site.
The next scheduled release is Firefox 43 set for around December 15th.