You can clear the already-stored cookies as described in this article: Clear your browsing history and other personal data on Firefox for Android.

Going forward, see: Enable or disable cookies on Firefox for Android.

That is not to say that all sites will work correctly with cookies disabled... if you don't mind storing cookies for the duration of the time you are working with a site, you can open it in a private tab instead.

I looked up your device and it appears to have a menu "button" on the device, at the lower right, next to the home button. Does Firefox recognize that button for calling up the menu?