Do you mean updating Firefox, or updating MacOS? For Firefox: The Extended Support Release of Firefox 115 is the latest version compatible with Firefox. You can check the precise current version using the About dialog. More info: * [[Firefox users on macOS 10.12, 10.13 and 10.14 moving to Extended Support Release]] * [[Update Firefox to the latest release]] For MacOS: You may be able to run a newer version of MacOS, but this is not considered a regular update to High Sierra, so the update checkers might not inform you. Apple calls it an "upgrade." According to an old Apple support thread, you can go to the version's page on the store and there is a compatibility checker feature. For example: * Monterey (12): https://apps.apple.com/us/app/macos-monterey/id1576738294?mt=12 * Big Sur (11): https://apps.apple.com/us/app/macos-big-sur/id1526878132?mt=12 * Catalina (10.15): https://apps.apple.com/us/app/macos-catalina/id1466841314?mt=12 * Apple support article: [https://support.apple.com/en-us/102662 How to download and install macOS] I suggest using Safari for this, since it may provide more intimate details about your Mac than Firefox can.