Большой русско-английский словарь - твердо

1. прил. кратк. форма от твердый
2. нареч. firmly, firm твердо стоять на ногах – to be steady on one's legs твердо стоять на своем – to stand one's ground (firmly), to hold one's own, to stand firm он твердо решил уехать – he is determined to go, he is determined on going
he has firmly resolved to go, he has firmly on going твердо выучить – learn thoroughly твердо держаться – stand firm/fast твердо запомнить – remember well твердо решить – determine, resolve (to do smth.
on doing smth.)

ad. firmly