Firefox 39 Rapid Release Cycle (RRC) will be released on June, 30th.
According to the release notes the browser has advanced security features and some performance improvements (for NPAPI Plug-in, dashed lines, etc.).
Mozilla Firefox 39 RRC (beta) has been successfully tested with NetWeaver versions 740 SP12, 731 SP16, 730 SP13, 711 SP13, 702 SP17, 701 SP16 on operating systems Win 7 64-bit, Win 7 32-bit, Win 8 64-bit (desktop), Win 8 32-bit (desktop),Win 8.1 64-bit, Mac and Linux in the areas of NW – PLS, WD ABAP, WD JAVA, NWBC, ACF, BI, UI5, FPM , Web CUIF, EP, Adobe, BPM; Fiori – FLP on ABAP.
As usual, for further details and SAP restrictions see: SAP Product Availability Matrix (PAM): https://apps.support.sap.com/sap/support/pam.
Firefox browser note: https://service.sap.com/sap/support/notes/990034