What do you mean by "stuff that I wouldn't want to do on Vivaldi"?
Vivaldi is set to be very strict when it comes to ads and tracking, and that affects sometimes how sites works. I've set Waterfox to be more "liberal"; it's for dirty-dirty stuff and dirty stuff. I'm also using it as a kind of sentiment I feel to Gecko engine. I'm not using Firefox since v60 - one of updates damaged my profile in an unrecoverable way.
Vivaldi is also a web browser: https://vivaldi.com/
Indeed, but I am unsure what OP would want to avoid doing on Vivaldi but would be happy to do on Waterfox.
https://news.ycombinator.com/item?id=20047170
https://news.ycombinator.com/item?id=15800634
https://news.ycombinator.com/item?id=10554083
And the one on acquisition by System1:
https://news.ycombinator.com/item?id=22338321
It's my choice for "sandbox" browser - as in doing all stuff that I wouldn't want to do on Vivaldi.