Forum Response - Platform version is not compatible message

This is a known issue caused by a corrupted installation after a failed update. The workaround is to uninstall and then re-install Firefox to fix this issue while our team is working on a more permanent fix. You shouldn't lose any data if you download the full installer from our official source.

See also: https://support.mozilla.org/en-US/kb/firefox-wont-start-find-solutions#w_xulrunner-error