Hi Are you using Firefox for Android or the desktop version of Firefox?

It's a acer Chromebook spin 713.

If you are using the Android version of Firefox (normally for say Android phones and tablets) then the extensions you can use are at https://addons.mozilla.org/android/ If you are using the Linux version of Firefox then the desktop Firefox extensions are at https://addons.mozilla.org/firefox/ Which you can use can depend on what you prefer though it can be limited by what ChromeOS version you have along with the CPU the Chromebook has. There are articles around including https://support.mozilla.org/en-US/kb/run-firefox-chromeos