Earlier today Mozilla made available security updates for Firefox namely Firefox 39 and Firefox ESR (Extended Support Release) 38.1 and ESR 31.8.
Firefox 39 resolves 22 CVEs while Firefox ESR 38.1 and ESR 31.8 resolve 21 and 17 CVEs respectively.
For an explanation of the term CVE, please see the first short aside within this blog post.
Full details of the security issues resolved by these updates are available in the following links:
Details of how to install updates for Firefox are here. Mozilla Firefox updates generally install without issues, however as always I would recommend backing up the data on any device for which you are installing updates in order to prevent data loss in the rare event that any update causes unexpected issues.