If you downloaded the Windows version (Firefox setup .exe), it will not work. https://support.mozilla.org/en-US/kb/run-firefox-chromeos There are also articles around about ways to install Firefox on a Chromebook though your options could depend on your CPU and ChromeOS version used. The basics is you have two main options you can either use the Firefox for Android or your can use Firefox for Linux as you can use flatpak or the Linux tarball from Mozilla even.

Is this an ARM chromebook or an Intel/AMD chromebook? *https://www.techradar.com/how-to/how-to-install-firefox-on-a-chromebook