A few things I like from #Firefox, over #Vivaldi, is the ability to have a bunch of user preferences defined in user.js, which I can then copy into new profiles so I don't have to hunt around in the UI to replicate a profile's settings.
userChrome.css and an organized chrome directory also.Vivaldi enables you to use a directory with a bunch of CSS files, but Firefox gives you userChrome.css, so you can \@import different files and keep things tidier.
The Firefox CSS modding community is also more active.
On the other hand, Vivaldi has a bunch of UI options that render some of the Firefox CSS mods pointless.
And yet, every time I think I'm settling on Vivaldi, there's something missing. The expand on hover animation of the vertical tab bar, for example, lacks a delay option, which I did find in Firefox, which I was able to set in user.js and copy it to all my other Firefox profiles.
Theming is also limited "officially" to 4 core colors. I was only able to expand them via CSS.
I love both, btw. I'll be using them side by side.