why can't I open fly frontier anymore?

There are two usual causes for this. One and the most common is old software, and is what I suspect is happening here.

Securing your data over the network is an ongoing effort among many folks, and it invariably means upgrading your own software, as well as other folks upgrading the software running on the remote servers we’re accessing. This means updates (patches, small fixes) and upgrades (new versions, larger fixes, enhancements) will be an ongoing project for all of us. Apple has been shipping yearly upgrades in recent times. These updates upgrades arrive as weaknesses and exploits are identified in existing and older software.

Yosemite 10.10 is too old to make a secure connection, and this is but one site that will be blocked.

El Capitan 10.11 is about as old as I’d suggest in recent times.

To upgrade, check your apps, check that your printer and scanner vendors have drivers available, get a backup or two, and upgrade to a more recent version:

How to upgrade to OS X El Capitan (10.11) - Apple Support

How to upgrade to macOS High Sierra (10.13) - Apple Support

How to upgrade to macOS Mojave (10.14) - Apple Support

Pick the newest version supported by your Mac and your key apps.

The other cause of these sorts of connection errors is installed (buggy, broken, sketchy) interception software, such as some add-on anti-malware and anti-virus apps, or add-in filtering tools, or (usually only on larger business, school, and organizational networks) network interception and monitoring tools. This case is less common for these reports than is old software. And Yosemite is getting pretty old.

This question was originally posted in High Sierra 10.13.

What happens when you try?

Maybe you are using the incorrect url.

Maybe you are running a third party app that is blocking access.

Maybe you have installed a browser extension that is blocking access.

Does this happen with all browsers?

Maybe the Frontier site is not functioning.

If you are using Safari try shutting down Safari then while pressing and holding the shift key relaunch Safari. This will launch Safari in a clean state.

What happens when you try to connect to FlyFrontier web site? Any error messages, diagnostics? What is the text of those?

High Sierra? Or some other release of macOS?

Any add-on anti-malware or anti-virus or network-monitoring-and-filtering add-ons?

Your own network, or a corporate or organizational network/

The problem with Safari (at least from my experience and perspective) is it stops receiving updates on older systems. Now I always use older hardware and systems (using Mavericks right now) but the one place I try to stay up to date is with a browser. Running Yosemite will not get you a new version of Safari. I use Waterfox which is a Firefox variant. I have no problems getting access to FlyFrontier. When I start up my older "up to date" Mavericks version of Safari I get the same error message as you. Bottom line, upgrade your operating system and associated Apple software or start using alternative browsers (I have 3 on my computer).

I would particularly urge you to use an up to date browser if you are doing things such as entering a credit card to buy an airline ticket. I don't care if somebody can hack into my feed I am using to browse the best brands of cat litter, but financials I take seriously. Even when my main browser was not an up to date version, I had a different one that was and for anything where I wanted security I would switch to that one for doing those things.

This turns into a can of worms about then considering upgrading either part or all of your hardware. Realize that technology advances and eventually you either advance with it or you stop where you are and it advances without you and cuts you off and leaves you behind. I simply do not understand why you must insist upon using an outdated browser version that will never be updated for the system you are running. The simple solution is to use another browser that does still work with Yosemite. If you really don't like it then just use it where necessary and go back to your old one. As I said before, I am definitely in the old hardware, old software school (I still use iTunes 7.5 -- yes, seven point five from 14 years ago) but the one place I will not compromise is doing what you were doing to produce this topic -- trying to buy something online.

Download and invoke and post the output of EtreCheck here, opening a new reply and then pressing the button that looks like a printed page to get a text input box that’s big enough to paste the entire hardware and software configuration report into your new reply here.

One of the usual reasons for problems with macOS upgrades is add-on software such as add-on anti-malware, add-on anti-virus, add-in cleaners, that sort of thing. The configuration report will show that. The integrated anti-malware tools and keeping patched to current work well for what’s generally around.

Whether or not to upgrade is entirely your call. Many folks prefer to stay on older versions and for a variety of good and bad reasons, but it’s entirely their call. The rest of the world will be moving forward irrespective, and network connections are one of those places where there will be upgrades necessary.

Staying on older versions gets progressively more difficult whenever networking is needed, and staying on older versions always gets more expensive.

Switching to a different add-on web browser will usually work for a while, but those browsers can and do and will inevitably drop support for those older macOS versions, too.

There’ll be other issues arising here, too, as much more than web browsing is dependent on establishing secure network connections,too.

Few of us like this situation, but it’s the world we’re in for now, and for the foreseeable future.

Only that one app? If yes I'd contact Frontier. Presumably some error on their server.