Uninstall Firefox from your device and then download install from the Mozilla Firefox website. When re-installation is complete, an automatic question will pop-up whether to keep Firefox as default browser, then click yes. It is always better to keep Google as your default search engine.

It’s very sad, but many of the 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.

Thanks @innu. I had tried re-installing Firefox, but I had never uninstalled it first. My mistake. I switched to the DuckDuckGo search engine because, although it can't match Google in overall performance, It's the only search engine that doesn't gather information about you and distribute it to unknown third parties. The times in which we live make that very important. Thanks @FredMcD, too. Your advice should be heeded by all. When it's not possible to download directly from the originator of the software, try a site like Ninite, which has earned a reputation for clean downloads.