Here is a Firefox Add-On that hides all ranks and ratings on OGS. It also hides the rating graphs and charts on profile pages, but you can unhide them by clicking on the spot where they should be. Some players might find it relaxing to not know their opponent’s and their own rank at all times.
There were already two older threads with this feature request and some pointers on how to do it, so I decided to just implement it. I hope that some of you find this useful.
By the way, the code is open source if anyone wants to port it to another browser.