firm 用作副词的用法十分有限，通常只能与 stand, hold, stay 等少数动词连用，且位于动词之后；而firmly 的用法则十分广泛，且可位于动词之前或之后。如：

Always hold firm to your beliefs. 一定要坚守信仰。

They stood firm against the war. 他们坚决反对战争。

I firmly believe that it is true. 我坚信那是真的。

The fence posts were fixed firmly in the ground.
栅栏的立柱牢牢地固定在地上。

They were firmly convinced that they were the rightful owner of the lands.
他们坚决相信他们是土地的合法主人。

【注】hold firmly 通常用于本义，指“紧紧握住”；而 hold firm 则通常用于引申义，指“坚持(原则、理想、信仰等)”。