I just installed Ubuntu on my MacBook Pro 9,2 (dual boot with rEFInd) and I'm experiencing constant drops of my WiFi signal.
I followed the instructions listed here Installing Broadcom Wireless Drivers to install drivers. I had to install linux-firmware-nonfree and firmware-b43-installer. My network issues persisted and I followed the debugging instructions on this page. It suggested that I ignore IPv6 so I did that but it doesn't help. I've been Googling for a few hours but nothing seems to work.
I ran the wireless script and here are the results.