I can't load a website that I've visited many many times in the recent past.

It is because of the second paragraph in my previous post. The Safari that you are using on Yosemite is blocked from connecting to modern websites that have upgraded their site security due to outdated security protocols, hence the messages, "can't establish a secure connection to the ____ server." The cure for this is a browser that supports current security (TLS) protocols. None are available for a 2014 - 2015 (Yosemite) Mac operating system.

As neither current versions of the Chrome or Firefox browsers support macOS Yosemite, you will need to upgrade your Mac to High Sierra or later, something that is determined by the age of your Mac and its hardware compatibility with newer operating system releases.

From your menu > About This Mac, take note of the content in parenthesis on the product description line. Compare the year of your MacBook Air to the hardware requirements in How to get old versions of macOS. Follow the link to the Mac App Store for the version of macOS where you Air meets the hardware requirements, and download that full installer to your /Applications folder. Do perform a Time Machine backup to an external drive before any upgrade attempt.

Hi stevecraig89,

Thanks for contacting the Apple Support Communities!

We understand that you're having some trouble opening websites you've used before in Safari on your Mac. We do see that you've already reviewed "Update or reinstall Safari for your computer" which is a great place to start.

Check out the additional troubleshooting steps you can take to help isolate this behavior further outlined here: If Safari on Mac doesn't open a webpage or isn’t working as expected

We hope this helps.

Cheers!

You should periodically clear your Safari browser cache and history content.

• Safari Preferences > Advanced > Show Develop menu in menu bar
• Safari application menu > Clear History… > [ all history ] > Clear History.

Sometimes, the content of those two entities above can interfere with browser load times. The other issue is if you are still using Safari on macOS Yosemite, it is years out of date with security advancements on websites, and because it is using truly ancient security protocols, you may encounter sites that simply reject your connect. And the third factor is out of date bookmarks where the site has changed their layout and the old bookmark borks.

If you are really still on Yosemite (macOS 10.10), neither current versions of Firefox, or the Chrome browser still support that operating system version and that drastically limits your ability to upgrade your browser alternatives with current security protocols.