Hybrid mountain bikes for the city? So I'm currently looking for a new bike and would like to get another mountain bike as it's most important to me that it's resilient and that I can ride all terrain.

However most of the time I'd use it for the city.

Are there mountain bikes (or are there ways for adjusting them) that are more fit for cities? In particular I'd like to sit comfortably on it.

I don't have much expertise with bikes which is why I'm asking here.
The standard advice if you want to ride a mountain bike on the road is to fit slick tires rather than knobby tires, and lock out any suspension if you have it (since you don't need it on the road). That being said, you're going to typically have lower gearing than a road or hybrid bike (due to the nature of mountain biking), though you may have some room to adjust this with cassette and chainring sizes. 

At the end of the day, if you're going to be using this bike only on the road, it isn't going to be as nice as an appropriately hybrid or cyclocross or road bike, and it would be preferable not to get a mountain bike. Note that hybrids/cross bikes are often suitable for things like fire roads, gravel and what not (light off road situations); they won't take well to dropping as much as a mountain bike, but you have to ask yourself if this bike is ever really going to do this (and whether you'd be better off just renting a mountain bike when you want to do this, or buying a mountain bike specifically for this purpose).