i have chrome on my Chromebook,can i download a different one on this device

So there wasn't one.

I have chrome browser on my Chromebook,can i download Firefox on this device

There are a couple potential options depending on your hardware. You can try the Firefox for Android from Google Play though it may function like you are on a tablet vs a laptop. The other possible option is to use the desktop Firefox for Linux version and there are a few options in installing.

There is this article https://support.mozilla.org/en-US/kb/run-firefox-chromeos and you can use a search engine to find other articles on install Linux version of Firefox on Linux part of ChromeOS.

There should be a version for Chrome,

Chrome is the web browser while ChromeOS is the OS. Mozilla does not have a version of Firefox made specifically for ChromeOS, whether a desktop or mobile version. If you have a newer Chromebook then you can potentially run Firefox for Android from Google Play or use the desktop Firefox for Linux in the Linux part of ChromeOS.