You should make Firefox feel native to each platform and follow its conventions, instead of trying to have a sort of unifying 'Firefox look' that feels out of place everywhere. Using platform-specific enhancements (like VideoToolBox on macOS for example) also would go a long way. Edit: most important of all is battery life though. Your browser can be super smooth, have all the nice extensions in the world and be FOSS, but if it cuts battery life in half everyone will hate it.