Definition - What does Firm mean?
A firm wine is one that has noticeable tannin structure. The tannins will tend to block any additional characteristics the wine may have. The structure also tends to lack complexity, but this is mainly for younger wines that need time to further develop. Properly bottle aging the wine can help.
WineFrog explains Firm
There are a couple reasons that a wine might be firm. If a wine is young, especially a red wine, this typically indicates that the tannin structure is overpowering. These tannins often block the ability to perceive other notes in the wine. However, with the right food pairing, tannin perception can be lowered so that fruit, spice and other aromatic and flavor profiles can be appreciated.
A wine may also be firm due to an imbalance in tannin structure. This usually occurs when the tannin structure from the maceration with the grape skins is different than the tannin imparted by oak aging. This is flaw which can not be corrected even with bottle aging, though some food pairings may help reduce the firm structure.