What kind of program is it? Is 'Relay' its full name? Have you tried doing a websearch?

It’s very sad, but many software downloaders/ installers will trick you into installing not only their program, but other programs as well.

You have heard of the fine print in shady contracts, right? Well, some installers you need to look at the itsy bitsy teeny weeny fine print.

You are thinking you are giving the installer permission to install the program you want by using the recommended option. But if you use the Manual Option Instead, you discover all kinds of stuff that you do not even know what it is or what it does.

From now on, everyone needs to Use The Manual Option to put a stop to this.

Note that these programs can also change browser/computer settings.

Make sure you select Settings > Privacy & Security > Permissions > Warn you when web sites try to install add-ons. Do you see it in Add-ons and Themes (ctl+shft+a)? If so, you can, hopefully, remove it there.

Perhaps you have this extension: https://support.mozilla.org/en-US/kb/activate-firefox-relay-hide-your-email-address-alias