Hi,
the Firmware is available in the Support Portal under:
https://extremeportal.force.com/ExtrProductDetail?id=01t80000001X8gtAAC
Login to the Support Portal, goto "Switching" not "Downloads", under "Stackable" you will find B5-Series.
The B5 went end of live 2017 but will reach end of support 5 years after that (Summer 2022). In doubt open a case with GTAC.
