I decided to try to contribute Firefox development, and they use Mercurial for source control. In the documentation about using Mercurial it was advised to choose a diff program and to do it before you start coding. They list a bunch of options, one of which appears to be the standard Unix/Linux diff program, which I'm somewhat familiar with via basic use.
The thing is, I hadn't realized that the diff program can even be an issue and I have a feeling that I won't know how to pick one until I pick the wrong one and get screwed by it somehow. So to skip this unnecessary hurdle, I'd like to ask what should I look at when choosing a diff program, and in what cases is the choice important (and in what way)?