I'm sure the @Vivaldi folks are sick of hearing this feedback, but I'll say it anyway (sorry): the only thing that holds me back from adopting and enthusiastically recommending Vivaldi is that it's not 100% open source.
With Mozilla chasing the AI dragon, now would be a really great time for Vivaldi to take the plunge and go fully FOSS. I don't know that it gains much, if anything, by being just mostly FOSS.

Thibaultmol 🌈
in reply to Joe Brockmeier • • •vivaldi.com/blog/technology/wh…
Why isn’t Vivaldi browser open-source? | Vivaldi Browser
Julien Picalausa (Vivaldi Technologies)Joe Brockmeier
in reply to Thibaultmol 🌈 • • •@thibaultmol I'm quite familiar with that post.
But it's been four years since it was published. They've had four years to assess whether being closed is accomplishing their goals, and a lot has changed with Mozilla in that time as well.
If I were at Vivaldi, I'd be arguing that the benefit of going fully open and welcoming Firefox users who are unhappy outweighs the benefits of being closed. Yes, people could fork the UI bits - but I've seen some pretty snazzy Firefox forks since that post that have really done wonders with the UI. I don't think the value of preventing forks is as high as they seem to believe. (Of course they don't have to provide their branding/trademarks in any source.)
Especially since AIUI Vivaldi has already struck a stance that is less AI-happy than Mozilla, I think they have an opportunity to win over some hearts and minds, as it were. They could also become the default browser and ship with distributions like Fedora and Debian, which won't ship it now as closed-source...
But, that's just, like, my opinion.