The Password Manager should not work until the Master Password has been entered.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

Hi Fred. Many thanks for your reply and sorry for my late one. I've realised that the account sync function does not cover the master password and I had only set up the masterpassword on one device, hence why it did not prompt me to insert the masterpassword on a new device.

Sync can not work if the passwords are part of the sync and a Master Password is in use. You must stop using the master, or not sync passwords.

I haven't tried on android as I'm using chrome there but I've tried between different PCs and it seems to have worked (though maybe my passwords have synced before I've introduced the masterpassword..?). I'll check back once I've got an update though according to this this thread it may have been solved.