I've modified your post and the topic's title to remove the profanity. There's no need for that.
This is actually a common thing that's easily fixed. You have three options that will fix this...
1) Reboot your computer/device (AdGuard for Windows *should* install the certificate into Firefox after a reboot).
2) In AdGuard for Windows' Settings > Network and press the green Reinstall Certificate button with Firefox completely closed. That *should* also install the certificate and fix this issue.
3) You can manually install AdGuard's certificate into Firefox by following the steps in this support topic: https://kb.adguard.com/en/windows/solving-problems/connection-not-trusted
If all else fails, follow the third option and manually import the certificate. That should always fix this issue.
So I bet you're wondering why does this happens in the first place? Is it AdGuard for Windows' fault? Actually, no it isn't. It's rather simple, Firefox uses its own certificate store and doesn't use the system certificate store like every other web browser does thus AdGuard's certificate has to be installed into Firefox. While this should happen automatically if you have Firefox closed while installing AdGuard for Windows, if you have Firefox open while installing this issue can happen. Without the AdGuard certificate added to Firefox's certificate store, you'll encounter errors like that when trying to visit HTTPS websites. In addition because of Firefox not using the system certificate store, this 'issue' also exists on Mac and Android with Firefox.
If you're still having issues after adding AdGuard's certificate to Firefox's certificate store, it *could* be related to something else, e.g. antivirus app incompatibility. If that's the case, the devs probably would need more information like what antivirus is being used.