On Android I'm using the open source 'Birthday Adapter' app which seems to work fine with Fruux. https://github.com/dschuermann/birthday-adapter/issues/81 It has a nice feature to set configurable reminders for birthdays as well as displaying them in your calendar.
As a workaround for this I use the following setup:
For desktop with Thunderbird use the 'ThunderBirthDay' addon: https://addons.mozilla.org/en-US/thunderbird/addon/thunderbirthday (There are also addons for birthday reminders)
On Android I'm using the open source 'Birthday Adapter' app which seems to work fine with Fruux. https://github.com/dschuermann/birthday-adapter/issues/81 It has a nice feature to set configurable reminders for birthdays as well as displaying them in your calendar.