Unable to update and install apps on macOS High Sierra 10.13.6

A Mac Pro (Mid 2012) desktop minitower cannot run anything higher than macS 10.14.* (Mojave) – and it can only run Mojave if you have installed a supported Metal-capable graphics card. For any of the MacBook Pro notebook computers introduced in 2012, macOS 10.15.* (Catalina) is the end of the line.

In either case, your Mac is too old to run any application that requires macOS 12.* (Monterey) or later. The current versions of most third-party Web browsers now require Monterey or later.

The current version of Firefox will run on Catalina, while Firefox ESR 115 will run on Sierra, High Sierra, and Mojave. Mozilla had planned to stop providing even "critical security updates" for Firefox 115 ESR a long time ago, but they kept postponing the time when they would abandon it. This time, they probably mean it when they say

"Mozilla will provide security updates for Firefox 115 ESR until the end of February 2026. After that, no further updates will be released and you are encouraged to upgrade your version of macOS."

Your OS is too too old. When OS's change it means developers might have to make changes to their apps which make them incompatible with the old version. Developers can choose whether to maintain apps for all OS's or just maintain them for the most recent ones. Looks like the Opera developers have decided to make their app for MacOS 10 and 11 obsolete - which is fairly normal behaviour as Apple tend to maintain the most recent 3 OS's. If your Mac won't update to 12 then there's nothing you can do about it.

There'll come a point when you'll start to find that some websites - particularly financial ones - are no longer accessible with whichever browser you use on High Sierra because they don't implement the latest security. High Sierra is also no longer receiving security updates from Apple. You've had 15 years' use from your Mac; that's pretty good but it might be time to think about a change.

well app makers can add requirements as to what macos version they work on.

it should be listed in app store for the app

or on the website the app was downloaded from

OS X and iOS apps minimum system requirem… - Apple Community

more

app macos requirements - Google-

By the way, High Sierra is eight major versions behind Tahoe, the current version of macOS.

Only a handful of Intel-based Macs can run Tahoe, and Apple has said that Tahoe is the last major version of macOS that will run on any Intel-based Mac.

Servant of Cats

..And there are signs from Mozilla Firefox ESR (high sierra, etc?) already

has allignments in posture, to dive-off board and leave more sites adrift.

Also marginal support of vintage Brave browsers has partially ceased on

Friday, Feb 13 2026; in early PM. Same message, with both; as I'd found

some versions could enter OK sites; as even Safari could be useful, too.

Hadn't attempted to 'self-certify' any the newer ones w/ 'Lets Encrypt.'

.While that had been a workaround where certificate expiry was pending.

Thought to add this Comment, had looked. Didn't find found any match.

Like the Blank progress bar: Nope!🍄