Long femurs. First, it helps to understand the problem, which can be so frustrating. For starters, fundamentally, in the squat the bar must remain over the base of support (i.e., in the space between the ball of your foot and your heel), or you tip over.
Holding constant everything else (bar position on your back, tibia and torso lengths, ankle and hip angles, etc.) the longer your femur, the further back the bar will travel (relative to your base of support) compared to where it would be if your femur was shorter. If you squat with an upright torso with any significant load, you get partway down and feel like you are going to fall backward, right? Right. Its physics.
What you often see with folks with femurs that are long, relative to the other two major levers (lower leg and torso), at least those who are not great squatters yet, is compensation for that backward bar travel with significant forward lean. (Frequently accompanied by some numbskull health club trainer in a polo shirt imploring them to keep an upright torso . . . shortly followed by the trainee adopting the leg press. . . .) Why do these folks lean forward? Because they are trying to keep the bar over the base of support, as they must. And that mechanical necessity leads to all sorts of other problems: (1) they cut the depth short, when they can no longer incline the trunk without rounding their backs (they sense they will tip over if they go “deeper,” which means the bar moving even further back); (2) worse, they compromise their back arch and round it; (3) with their trunks leaning so far forward and the hip angle getting tighter and tighter, their tight hamstrings win the stabilization battle with their erectors, and the pelvis tilts under (the so-called “butt wink”); (4) their heels come off the floor, because they are tryig to incline the shins (flex the ankle joint) in an effort to get the load back over their feet; or some combo of the above. All of it can be driven by the physical fact of femurs that are significantly longer than the tibia, torso, or both.
It can be dealt with, and good squat form achieved, for long-thighed folks, even those with femurs several inches out of whack relative to the other two major levers. Some combination of the following usually do the trick.
Widening the stance, and/or turning out the feet. (Picture yourself squatting inside a 24" deep box, feet hip width, toes straight ahead - your butt hits the walls a quarter way down - but if you widen your stance and turn your toes out 45 degrees, you can squat without hitting the walls.) How wide and how much turnout you can use will be limited by your hip architecture and flexibility - for many people, at some point your hip joint will feel like it is “binding up” (because it is) and you won’t be able to get deeper. But some widening and turnout is almost always necessary for long-femured lifters.
Heels in your shoes, and/or improving ankle mobility. Heels, as in Olympic lifting shoes, make your tibia effectively longer relative to your femur and torso lengths, so that any given degree of ankle flexion translates the load further forward, balancing out the excessive rearward translation that your long femurs create, and so allowing you to stay over the base of support without so much forward lean in the torso. More ankle mobility does the same thing. Its why Olympic liftes wear those shoes - they have to be deep, with upright torsos, or they dump the bar. You’ll squat better when you can push through the entire foot, from heel to ball, without doing the tippy-toe, plates under the heels thing.
Improving your hip mobility (especially ham and glute groups). Unless you have a long torso to match your long femurs, you’re going to lean forward more than somebody with short femurs or evenly matched levers, because, again, you have to do so to keep the bar over your feet. That forward lean and increase hip angle magnifies the tension contest going on between your erectors (working to keep your lumbar arched and your pelvis tilted one way) and your hamstrings (which are getting stretched further and further and pulling on your pelvis in the opposite direction). To maintain your correct, tight neutral spine in the hole, you have to have enough flexibility in the hips to prevent your hamstrings from surpassing the tension your erectors and causing your butt (pelvis) to tuck under. Look up “Agile 8” - a hip mobility sequence on Joe DeFranco’s site, and the mobility and stretching exercises on the “Squat Rx” channel on YouTube or the site of the same name. Effective stuff that you can do everyday and see major improvements in a few weeks time.
The fix(es) are to (1) work on your glute-ham and ankle flexibility (goblet squats, squats holding onto a pole or the side of a power rack, Agile 8, etc.); (2) experiment with widening and turnout of the stance, finding the best tradeoff between whatever depth you need to achieve for your goals, balance, and hip mobility - somewhere between just outside the hips and sumo-ish wide will work best - find the sweet spot for your anatomy and goals; (3) if you can, and you know your tibias are short relative to you femurs, try WL shoes with a 3/4 to 1 inch heel; and (4) most important, practice squat technique as frequently as possible, going as low as possible, with the best possible form, sitting in the hole as deep as you can get without losing your arch. The best mobility exerise for the back squat is . . . the back squat. Just squat a lot, throughout the day, with or without barbell, working on getting deep, with an arch, and your feet flat on the floor. Improve a couple of millimeters every day, and in a month your solid form will be two inches plus deeper.
Since you’ll always have a little more forward torso lean than your short-femured brethen and sistren (which is fine, as long as you maintain a tight arch from ass to brainbucket), of course make sure you are making your erectors and hip extensors strong and mobile - good mornings, back extensions, RDLs, all the usual stuff that you should be doing anyway - and those too, done with absolutely no compromise of the tight neutral spine / arch.
Lots of people overcome this anatomical issue and become accomplished squatters, powerlifters, and Oly lifters. It is rarely a deal breaker for the persistent - just takes some understanding of the lift, and a few weeks to a couple months worth of dedicated effort to optimize your form. (The payoff is a lifetime of superior lower body strength.)
Hope that helps.