I have a long-standing back problem and find that what helps me the most is good lumbar support. In both my current CX-5 and in its predecessor, a Mk2 Ford Focus, there is an adjustment for this and I set it to give the maximum. It ensures I'm sitting upright and, while sitting in the same position for a long time is not good for the back, it enables me to do long journeys without major issues.

Make sure you take the Mazda on an extended test drive before buying if you have back problems.

I don't find the seats great for the lower back on long journeys, and the lumbar support is of the "bar in the back" variety, I preferred the electric height and depth adjustable lumbar in my Golf MK5.

Whatever you do, and you do need to check you've got it right on your current car - make sure you adjust the seat and steering collumn position so you aren't stretching to reach the steering wheel at all, and centre console buttons that much, when wedged into your seat. Same goes with depressing the pedals.

I found on my first car, a mid 90s Nissan Micra, that my seat was one notch too far away - I could still reach everything, but my arms were straight when I held the top of the steering wheel, and I has pivoting with my lower back when fully depressing the clutch, causing me to have serious back problems when using the car on my longer journey to work (20 miles each way) at that time. I never had any problems when I was doing low speed, short distance driving to my previous job. With the new seating position, all my back problems went away.

Besides any possible inherent poor design of the Fiesta, I would check the rake and height adjustment (if it has one) of the seat, any lumbar setting it has too, as well as the ones that make most difference - the seat distance from the steering wheel and the height and reach of the steering collumn. Its not always the case that just adjusting the seat position works - I found an even more comfortable driving position in my 2005 Mazda3 as it had reach as well as height adjustment on the steering column - my arms are relatively short compared to my leg length, and so a small adjustment helps.

What won't help, especially for non-luxury cars that have seat/steering collumn memory settings is that if you share the car with others and if one/more of them driving is significantly different in height.build as you, re-finding the correct position for everything is difficult, hence why you may find its not to your liking. Sometimes its worth paying that bit extra for a car that does have that facility in such circumstances.

One final tip - whether you've already adjusted the above to what you think is the best driving position for comfort or you wish to have another go, once you've changed it, give it a while (a couple of weeks as a minimum), then if you find doing certain things (e.g. steering, pressing the clutch fully, something else), adjust ONE THING, then leave the rest for another fortnight, then adjust the same item again or another as required. Adjusting too many at the same time means you may not know what has worked and what hasn't.

Best of luck.