I run OS X El Capitan on a Unibody MBP 13-inch mid-2009 and support for El Capitan is likely going to run out in fall, so as El Capitan is the last OSX update supported for my MBP, I will stop using OSX in a few months. The MBP still runs fine though hardware-wise so I’m thinking of just using it with Ubuntu. I’m just wondering if this would be unsafe as I can’t get any updates anymore for Mac OS X or if it doesn’t matter and I’m fine using Ubuntu security-wise? Thinking of things like maybe firmware updates that would have to be tailored to MBP hardware and supplied by Apple (but I’m really just guessing). Any advice is appreciated, thank you!
This should be fine.
A mid-2009 MBP won't be seeing any firmware updates from Apple.
If you ever found some reason you needed to run macOS on the machine, you can boot from a USB stick. Consider prepping that USB stick before you switch to Ubuntu.
Linux can definitely help breath new life into an older Macbook. You may want to consider a lighter-weight desktop environment or window mananger to maximize that benefit, though.