Is it safe to put a front-facing car seat at the front passenger seat? Do I have to disable the airbag first?
First off, this is not medical or legal advice, and no answer from a site like this should be relied upon; ask your pediatrician for the best advice.
It is never safer for a child to sit in the front seat. (Similarly, it is never safer for an adult; the rear seat is simply safer in general.) The recommendations of child safety organizations are generally they should be in the back at least until 13, if not longer (see this page for example).
However, if your child must ride in the front seat, disabling the passenger airbags is certainly a must. You may be able to switch them off, and if not your car dealer or mechanic may be able to install a switch to turn them off.
Some additional information, from the American Academy of Pediatrics, who published updated recommendations for car safety for children:
There are a number of studies mentioned in the article, some fairly recent, backing those numbers.
Yes it should be safe, and no, you don't have to disable the airbag in most countries, although you'd have to check the laws in the country where you live.
According to this official Dutch source (in Dutch), children under 18 and smaller than 1,35m. have to use an appropriate car seat. Rear-facing car seats are only allowed on seats (front or rear) without an active airbag (so it should either be deactivated or absent altogether), but front-facing car seats have no such restrictions. This applies more or less to the entire EU, although some countries use different height or age restrictions, as evidenced by this official UK source, that puts the age limit at 14.
As I expect these laws to be based on safety, I assume it's safe enough to use a front-facing car seat in the front seat.