Protingai tvirto būdo žmogus baimintųsi dėl savo saugumo.
Šiaip terminas mūsų teisėje nėra nusistovėjęs.

There is a little bit lack of a context, but I think that the expression „a person of reasonable firmness“ is a very formal (formally neutral) meaning „a person of moderate abilities“ (physical or any other abilities/capacities that are not enough for the person or the person feels that these are not enough to be safe). In very simple words, e.g. if you are an elderly person and you think you cannot defend yourself in case, e.g., of a robbery threat (robbers are „firmer“), you will feel unsafe/frightened/scared.

So, my suggestion of translating this would be:

Žmogus, (objektyviai) vertinantis savo (fizines/psichologines) galimybes kaip vidutines, jaustųsi nesaugiai.