Any exercise that uses that particular muscle will help to build firmness there if you do it enough to exhaust the muscles of the inner thighs.
In Pilates, which I do, there are exercises where a medium to large size fitness ball is placed between knees or ankles or feet, and you have to SQUEEZE the ball while the legs move (up and down, etc). Squeezing to keep the ball from falling uses those inner thigh muscles.
If you lie on your side to do leg lifts, but you lift the bottom leg UP whether the other leg is down or the other leg is elevated....you LIFT the bottom leg up and use that inner thigh muscle.
If you do ballet barre exercises and have to move the leg INWARD (saw, you're sweeping it in semicircles or across the front or back of body), you're using the inner thigh muscles.
If you just SIT i a chair watching tv and put a smaller size ball (one that has give and you can sqeeze), and just squeeze/release, squeeze/release, etc, you use inner thigh muscles. So, forget thighmaster (awkward to use, I know, cause I had one). Just get a medium size fitness ball and you can use it for all sorts of other exercises, not just inner thigh. By medium size, I mean not the huge ones you sit on or the tiny ones you do arm exercises with. One that you can hold comfortably between your knees and have enough spacing that you are at ease and can squeeze away. If you want to start doing it right away and don't have any balls, etc: Just use your hands and arm strength to work again. Put your hands flat against your inside lower thighs/knees, and as you push in with thighs, RESIST with your hands/arm strength. The point is to activate the desired muscles and work the to exhaustion/burn.
If you just sit and use them minimally, you will get negligible firming. If you aren't sore the next day in the region worked, you just didn't work hard enough to get the results you want
Make sure you always engage your core muscles to stabilize so you don't arch/strain your back.
And Google is your best info friend:
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