Are you trying persona account?

What's the error your getting when your try to login?

Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache. The Firefox cache temporarily stores images, scripts, and other parts of websites while you are browsing.
Note: This will temporarily log you out of all sites you're logged in to. To clear cache and cookies, do the following:

1. Tap the menu icon located at the top right corner. This is the icon with 3 bars. On older Android devices you'll have to press the hardware menu key and then tap More.
2. Tap Settings.
3. After that, you will be taken to the settings screen. In the settings screen, look under the section Privacy & Security and select Clear private data.
4. You will then be taken to a list of what can be cleared. Select the following 2 for deletion:
   • Cookies & active logins
   • Cache
5. After those have been selected, tap the Clear data button to actually clear the cache and cookies.

Did this help you with your problems? Please let us know!

I have done what you suggested above and it does not allow me to sign in and set up sync

Is it working in Desktop?

If you set up Sync a long time ago you will need to create a new account and set up Desktop and Android Firefox to use the new Sync Account. See https://support.mozilla.org/en-US/kb/how-to-update-to-the-new-firefox-sync