If I don't need functionality of base qooxdoo class qx.core.Object could I inhererit my own qooxdoo class (extend) from Javascript Object class. What are pros and cons?
It depends on what you want to do. Qooxdoo is able to use code from NPM packages anyway. Those packages have their own code, some times with their own classes.They are not aware of Qooxdoo in any way. One pro I can think of is that you can create your own NPM packages to use in other projects that don't involve Qooxdoo. The con is that you loose what "qx.core.Object" brings with it and the tighter integration with Qooxdoo.