For Firefox and Thunderbird it's just as easy except you just extract it from the archive, then you run. Many apps don't need to be "installed" while some (usually non-cross platform ones) rely on external native libraries that my not be generally available or distributable with the app. That's what Yast and RPM are for; resolving required binary library app versions to provide consistent behaviour. There will be a learning curve with any system, but intuative usability also depends on the expectations of the user. For me certain aspects of Windows are very unintuative and unusable because I haven't used it in a long time. The issue many people have is that they only want to learn how to use Windows and expect that to be the standard interface to everything.