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Her weapons, in her human form at least, are limited to a knife and a spinning blade.
The knife is the most commonly found weapon, and you can hold as many as 3 at a time. It is possible, through some glitch, to create four, five, or possibly more knives, by throwing and catching them rapidly. However, the extra knives will float around and fall to the ground, unless you throw one to free up a space in your inventory. You can have up to three spinning blades as well, but it is unclear if it's possible to "create" more. At only one point in the game do you actually have access to 3 at one time, and this occurs in the final level of Jill III. After throwing the spinning blade, it does not "return" to you, and so, there is no possibility of not "catching" it. After the blade either strikes an object, or reaches its time expiration, it disappears from the screen and returns to your inventory.
The spinning blade can destroy certain bricks, which might impede your progress or conceal a key. It is the easier weapon to use, as its killing surface is bigger, it is simpler to aim, and you don't have to worry about its recovery. With the knife, it is possible to lose it, because it could fall into a deadly tile, such as water or spikes, or it could get stuck in a 1x1 square, making it impossible to recover. In the firebird and fish form, the weapons utilized are in unlimited quantity, and function more or less like bullets, shooting straight, and with no need to reclaim them.
As a firebird, you will shoot fire rings. Interestingly, when you shoot them, you move upwards, as if you "flapped." It is also worth noting that the rings can pass through one square of a wall, sometimes a useful strategy. Finally, the fire ring is the only way to kill a ghost.
As a fish, you will shoot "fish bullets." Not much to say about these, except that they seem to be effective only against water enemies.