So was that a addon or what the the FF language version that matches the installed languaged used?

A Google search comes up with this Tatar dictionary for Firefox and Thunderbird.

• https://addons.thunderbird.net/en-US/thunderbird/addon/tatar-spellchecker-dictiona/

Is this the extension you used to install the dictionary or did you install the dictionary otherwise?

This dictionary is quite old (2013) and might not be compatible with current Firefox releases because the extension isn't signed.

In Firefox 62+ there have been changes to the way dictionaries are installed (builtin dictionaries are moved to omni.jar) and some code to unpack a dictionary has been removed.