That's great! Could you please provide more details about what you're considering between Waterfox and Floccus? This will help me give a more accurate answer.
Floorp's Vertical tab bar is an experimental feature, which I've tried but is broken. It only opens one tab out of the eight I have it set for when using the Horizontal tab bar, which works fine. I've never used Waterfox.
If I things as far as vertical tabs are equal, Waterfox's development is stagnant compared to floorp. Having used both, I can say without a doubt floorp is just better; it has more features and faster development. I can't say I trust Waterfox having a single developer. They're both based on Firefox ESR, so you're not losing out on any fundamental features by choosing either one.
As already mentioned by @wat0114, Floorp is the only one, but the feature is still in development.
AFAIK, for all other Gecko based browsers there are no built-in feature for vertical tabs,
You need to install browser addons/extensions to use Vertical Tabs, but be aware you have to fiddle with...
This may not matter any more, but v12 of floorp with no longer be based on Firefox ESR but rather a rapid release like Firefox or Librewolf. It'll be up-to-date with the newest features and patches.