I installed a fresh Ubuntu 14.04 on an old MacAir (how to do that is explained here) and took me hours to get it to see the Wifi networks. There were many suggestions on many sites but the solution that finally worked for me was Hadaka’s answer from ubuntuforums

Just for information, check your pci id via

lspci -n | grep 0280

Then some cleaning up

sudo apt-get autoremove
sudo apt-get remove --purge bcmwl-kernel-source  #for good measure
sudo apt-get remove --purge b43-fwcutter firmware-b43-installer

sudo rm /var/lib/apt/lists/*
sudo rm /var/lib/dpkg/lock
sudo rm /var/cache/apt/archives/lock
sudo dpkg --configure -a

sudo apt-get update # I didn't do this as I didn't have a network

Now the important part is:

sudo apt-get install linux-firmware-nonfree

however, as I didn’t have a network I downloaded linux-firmware-nonfree_1.14ubuntu1_all.deb to a USB stick (you might want to get a newer version) from here to a connected computer and then

sudo dpkg -i linux-firmware-nonfree_1.14ubuntu1_all.deb

and then

sudo modprobe b43

Finally, rebooted and was able to see and connect to WiFi networks