The most common and most likely answer to this crossword clue is the 9 letter word UNCHANGED
I've seen this crossword clue in The Daily Telegraph
U
N
C
H
A
N
G
E
D
More clues leading to the result UNCHANGED
Buddy explains!

The straight part
We know that "Firm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case firm = unchanged.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does UNCHANGED mean?
UNCHANGED verb- To revert or reverse a change
- To not change; be unchanging; remain constant
- Not changed or altered; remaining in an original state.