Thank you. It worked. Copying the User Data directory didn't want to work for some reason, but cutting and pasting did. I started Vivaldi, closed it and ten moved the files back. The browser works much better now.
@hackminded No problem. Yes, that's correct. Quit Vivaldi using either "Command-Q" on the keyboard or by clicking "Vivaldi" on the macOS Menu Bar and then selecting "Quit Vivaldi" from the dropdown menu.
... and welcome to the Mac! FYI, Apple has some great tips on their web site to help familiarize you with macOS and to ease your transition from other operating systems.
@ryofurue FYI, the Firefox developers have been working on something that they call Containers as a more elegant way to solve your particular problem. It's been a work-in-progress for ages but now seems to be getting even more focus.