High contrast mode considered an important accessibility feature. Formally we have always supported this, but in reality our support has been far from perfect and in many cases gotten worse over the years.
Here are a few screenshots to illustrate what kinds of bugs we've fixed:
Menu bar, search bar and about:home (before): http://imgur.com/vMXgZMm
Menu bar, search bar and about:home (after): http://imgur.com/a4Volo5
Menu panel and customize mode (before): http://imgur.com/iBXWbeA
Menu panel and customize mode (after): http://imgur.com/n881M4v
As you can see, issues range from text being completely illegible and icons disappearing, over icons and text lacking contrast, to some parts of the UI not adjusting at all to high contrast themes.
Support still isn't perfect but a lot better than it used to be. Going forward we as a team will need to remind ourselves that high contrast mode is a thing and that we're committed to supporting it. Please keep this in mind when writing and reviewing patches. Please test your features in high contrast mode.
You might want to take a look at the fixed bugs to get a sense of what's needed to support high contrast mode. It often comes down to picking platform colors instead of hardcoding color values, or carefully picking color values that don't assume that everyone is using the same default OS theme. If you're unsure what's needed to make your feature properly support high contrast mode, feel free to ask me for feedback.