Available OS updates for iMac 5K, 27 inch, Mid 2015
What is the latest update available for iMac 5K, 27-inch, Mid 2015? Currently running Big Sur 11.7.70 and will not update from this version.
Big Sur is the latest major version of macOS compatible with that iMac (Retina 5K, 27-inch, Mid 2015).
It looks like macOS 11.7.11 – which came out on February 2, 2016 – is the latest release. It provides an updated security certificate needed to keep iMessages and FaceTime (among other things) working after January 2027.
MacWorld – Apple releases surprise Big Sur update to keep FaceTime and Messages working
Apple security releases – Apple Support
Most major third-party Web browsers now require Monterey or later, but the current version of Firefox supports Catalina and later, and should run on your machine. You may want to install it, given that it has been a few years since Big Sur received the latest versions of Safari.