Earlier today Mozilla made available Firefox 43 and Firefox ESR (Extended Support Release) 38.5.
Firefox 43 resolves 24 security issues more formally known as CVEs (defined). This includes 3 high severity issues (not yet assigned a CVE). Individually the severity of these issues is as follows:
5x critical severity CVEs
7x high severity CVEs and 3x high severity issues
6x moderate severity CVEs
3x low severity CVEs
Meanwhile, Firefox ESR 38.5 resolves 9 security issues (8x CVEs and 1x issue not assigned a CVE):
4x critical severity CVEs
4x high severity CVEs and 1x high severity issue
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. If Firefox is your web browser of choice, please update it as soon as possible to resolve these security issues.
In general, Mozilla Firefox updates 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.