This should help, also - Control whether Firefox automatically fills in forms

Type about:config in a new tab then type in or copy paste:

WARNING: Changing preferences through this interface not officially supported Hidden settings edited using the about:config tool are explicitly not supported, which means that Mozilla makes no guarantees they will be supported in the future, or that Mozilla will fix them if they break. Mozilla does not test these preferences, and will not in the future. That includes security and performance testing which these preferences may affect.

[Warning added by moderator]

extensions.formautofill.addresses.supported and change from detected to on also check extensions.formautofill.addresses.enabled set to true if its not already