Changing engines between Opera and MicroB:

Open the browser and use the top left menu. In the list you will find engine options MircroB and Opera. Make your choice and than immediatly close wour browers (all) in the same menu. Next time when opening a browser it will use the engine you choose.

By the way: Most of the time I use Opera. Faster and less bugs then the MicroB Beta version I installes.