Yes, you can.
Chrome OS devices run Google’s Chrome browser & nothing else by design.
OK, some of them now run Android apps, so I guess you can install the Android version of Firefox on one or two Chromebooks at the current time, but the rest are definitely Chrome only, unless you switch them to developer mode and install a different OS.
Not even close to answering the original question, but technically, you can install Firefox for Android on some Chromebooks.