Here's how Mystery Gifts work in FireRed:
* Through a physical Game Link Cable: You could trade with someone who has a Mystery Gift item. This is how many players got their Mystery Gift Pokemon back in the day.
* Using the Nintendo Wi-Fi Connection (no longer available): This was the main way to get Mystery Gifts. You would connect your game to the internet and download special content.
* Through an emulator: Certain emulators have built-in features that allow you to add Mystery Gifts. This can be a bit more complicated and may require some searching for specific guides.
Unfortunately, due to the Nintendo Wi-Fi Connection being discontinued, the official way to get Mystery Gifts in FireRed is no longer possible. You might be able to find some people still offering Mystery Gift trades online, but this is often risky and may not be legitimate.
If you want to experience a Mystery Gift in FireRed, your best bet is to use an emulator and find a reliable source for adding them.