Vivaldi-to-Firefox here with a little insight! Firefox addons can have the permission to hide tabs, and there are addons that take good advantage of this. Simple Tab Groups can essentially replicate Vivaldi Workspaces, as well as Sidebery if you want something a bit more on steroids in the form of a sidebar.

~~While I've got you here, I've had issues with Vivaldi not being able to block Google search results I don't want to see. Might've been me not setting my blocklist up properly, but it works on Firefox with uBlock Origin and Safari with Adguard.~~ Seems like Vivaldi doesn't support some of the more advanced filtering that Letsblock.it uses for that - AdGuard works.