Did you just start Firefox and was asked to install an add-on called Set Search Settings 1.0?

If so, you have probably downloaded and installed some software that bundled Set Search Settings 1.0 in the installer. I’d recommend a scan with FreeFixer or some other tool to examine and clean your machine from those potentially unwanted programs.

If this helped you figure out how Set Search Settings was installed on your machine, please post the name of the software that bundled it.

Thanks for reading.