I know many of us still wish to use Flash after EOL to customize our pets, play our favorite games/dailes, and grab a few more avatars!
I have managed to get Flash working using a similar method to a reddit post that showed me how to play Shockwave games: (you need an account to see links)
I will be reiterating the method that worked for me to get Shockwave games working (except Castle Battles) and explaining how to do the same with Flash.
Forenote: This worked for me on macOS High Sierra 10.13.6. The links I will include will be for macOS. For other operating systems (and newer versions of macOS if this method does not not work on those), I recommend using a VM running macOS High Sierra 10.13.6 or finding corresponding downloads for your OS.
Warning:
There are inherent security risks when using an outdated browser and outdated plugins, as this guide suggests to continue using Flash and Shockwave. Do NOT navigate the web freely when using the outdated Firefox and corresponding plugins. Usage is only recommended for official Neopets pages like games, HSTs, dailies, and customization. It is recommended to stay away from user lookups, pet lookups, pet pages, shops, galleries, and the neoboards entirely.
Guide
Downloads
The first step is to download (do not install immediately after downloading) the older versions of Firefox, Flash, and Shockwave.
Old Firefox, per the linked reddit: (you need an account to see links)
Old Shockwave, per the linked reddit:
(you need an account to see links)
Old Flash, a newer version may also work, I went back an entire version to ensure the killswitch hadn't been encoded yet:
(you need an account to see links)
Installation
Use your computer's internet settings to disable internet connection before proceeding. Do not re-enable your internet connection until the end of the installation section.
Once your internet connection is disabled, install all three downloads. They should not require an internet connection to install
Proceed to open up Firefox, navigate to "Preferences" from the start page, click on "Advanced Settings," find the "Update" tab, and set the "Firefox Update" setting to "Never check for updates." Confirm your changes.
Next open up macOS System Preferences, and Flash Player should be on the bottom row of icons. Again, click on the "Updates" tab and set it to "Never check for updates." Confirm your changes.
You can now re-enable your internet connection.
Usage
Firefox will require you to "Agree and Remember" to use an outdated version of the Flash plugin. This will pop-up as a Red Lego icon with a slash through it in the URL bar when you try to access Flash content for the first time. I have had to "Agree and Remember" everytime I reopen the browser application after closing it.
Firefox should also require you to "Agree and Remember" for Shockwave initially. It has not required me to "Agree and Remember" everytime I open and close the browser application.
If a Shockwave game get stuck on the game's loading screen with the classic Neopets game loading screen looping, simply close the pop-up window and click play again until the start menu loads. I have been able to get all Shockwave games working excluding Castle Battles.
Thanks for reading and enjoy!
PS - Some on the Neopets users, per the neoboards, have continued using their current browser and the latest Flash plugin by changing the date on their computer. I would not recommend the date changing method for long term usage as it can invalidate security certifications and mess with antivirus updates.