That message about missing firmware means the kernel is loading the b43 driver for your device. Some versions of the BCM4312 can use the b43 driver, but it requires firmware that you must acquire from elsewhere and install. To save you some time, here are the exact steps to try if you have a wire connection to the Internet for Fedora. Open a terminal and enter these commands one after the other. You can copy them directly from this page into your terminal...
tar xjf broadcom-wl-184.108.40.206.tar.bz2
b43-fwcutter -w /lib/firmware wl_apsta_mimo.o
After that, reboot and check NetworkManager for networks.
If you do not have an Internet connection in Fedora, then download that tarball on some other computer, somehow transfer it to the Fedora filesystem, and pick up the steps with the tar command.
If the commands above completed successfully and the wireless still is not working, then return for some suggestions to try.
If you never get anywhere with b43, all versions of the BCM4312 work with the Broadcom 802.11 STA Linux driver (aka broadcom-wl). I will help you try that next if b43 bombs with this card.