Please note that the current version of Firefox is version 89.0.1, released on June 17, 2021, which requires macOS 10.12 or newer. That means that you are running an older version due to the fact that your operating systems are preventing you from using a newer release. I don't know how old your MacBook and iMac are, but you could consider uprading macOS on both if possible, that may get you access to a newer version of Firefox as well.