If your two firestorms don't stand a chance it means it's very late game and UFOs have probably maxed out their HP and weapons, and you're running late on building Firestorms and better weapons. Early-mid game I took down large UFOs with Phoenix cannon and sparrowhawks. Those were good times... But even if you max out everything, you still won't dominate.
I'm guessing you're out of modules too.
The thing about the air game is that there isn't anything you can do on short notice. It's all about planning. Like for example, I try to keep at least 100 floater corpses in reserve, so I can convert them to aim modules (I always try to have 20 aim modules ready to go). Aim modules are a must have especially with EMP/Fusion. I also keep all disk wrecks so I can convert them to defense modules, but those are more rare so I try to use them against very large only.
Here is the big picture. The air game will always suck. It's meant to, even after you max out everything. You are not meant to shoot down every UFO. Get used to letting UFOs go. Get used to even losing continents, and then getting them back.
Pick your fights. In fact, you want to let go some of the UFOs on purpose.
- Let harvest/research UFOs land, so you can raid them for resources.
- Let abductors land so you can get an abduction mission which is great for reducing panic and getting lots of meld.
- Focus on attacking UFO that increase panic, e.g. terror, air raiders, satellite hunters.
- Even if you don't shoot down a UFO, simply damaging it may force it to cancel it mission. If you damage it 50% or more (you'll start seeing explosions), the UFO will leave without completing its mission.
- Cycle interceptors between the continents (Core strategy). Late game UFOs tend to focus on one or two continents. Any jet that's damaged for more than a few days should get cycled out to your 'reserve' continents that don't get much action, and replace it with a fresh one (2 days transit). This of course means you should always keep at least 1-2 empty jet slots which gives you space to transfer them. E.g. it's much better to have 29 interceptors that you can cycle, than having 30 of them that you can't cycle between continents.
- Letting a hunt UFO take out your satellite is not that bad, as long as you keep a few spare ones, and can put up another one before the end of the month, or right away. Sometimes I don't even bother attacking hunters if I can replace the satellite right away.
- Losing a continent is not too bad if your ground game is fine. In fact that can be used as a tactic to reduce alien resources. Aliens use up a lot of resources to build alien bases. If you assault and take back a continent right after you lose it, it costs the aliens dearly.
- The one time I definitely try to take out a very large is when they fly toward my HQ. I hate HQ base defense, so I try to prevent it. Btw keeping alien resources to a minimum will delay HQ defense.
Beating the game doens't mean killing everything you see. It's about strategy. Learn the UFO stances, so you'll know what to expect and how to react to them. All of this and more is explained on Ufopaedia...
https://www.ufopaedia.org/index.php/Air ... (Long_War)
https://www.ufopaedia.org/index.php/UFOs_(Long_War)
https://www.ufopaedia.org/index.php/Ali ... (Long_War)
p.s. Re: szmind and Squadron Unleashed. That mod may be fun to use, but it's no longer Long War. The air game in LW is meant to be difficult and strategic, and that mod tends to take away the strategy component or change it too much. It's good to have the option of course, for people who want easier air game.