I used to frequent the K-Meleon fora and we've had the MDI/SDI issue has always been one of the biggest. I, along with the K-Meleon developers, believe that SDI is the better option for our purposes. However, there are a lot of people who seem to like tabs. K-Meleon's position, as I understand it, is that the developers don't want to implement tabs because that's not something they're ever going to use. However, if somebody else wanted to implement tabs as an option, they'd keep them in the program.