I also prefer the older viewer. In a different thread I was linked to a work around to keep the old viewer while using the new version of Calibre, thanks to the very helpful Sirtel. Hopefully there will be more customization options in the future to make things easier for those that preferred the old viewer, but this workaround is great in the meantime.
https://www.mobileread.com/forums/sh...1&postcount=47