YAY on your first Tofu cooking! Looks GOOD! (And as resident Tofu Princess, I compliment you on a job well done..)
As others suggested, there are ways to make the tofu more "firm" - the first (though not so healthy) is to fry in oil in a non-stick pan (I use my wok) till it's yummy and browned.. the second suggestion, as listed above, is the cornstarch/soysauce mix - it makes a nice gooey brownness on the outside of the tofu and it does seem to get firmer too... cooking longer in the sauce will flavor it more, but won't make it any firmer - marinating ahead of time can help w/ flavor too..
Tofu's actually pretty easy to work with once you jump in and try it...
Have you tried some fresh ginger and crushed garlic in your veggies and tofu? Easy additions that TOTALLY make the flavor explode!! A friend suggested leeks instead of onions and I am totally hooked on those now too... not critisizing your dish - it looks and sounds PERFECT - just throwing ideas out for next time...
PM me if you want any tofu recipes, I have tons!