silvenga Hey there!
That's interesting, as I'm unable to select Firefox as search app on my Pixel?
If I long press the search widget and open settings, I do get a menu to select my search app. But Firefox simply doesn't appear in the list. Not the regular one, not Firefox Beta and not Firefox Nightly either.
I also tried disabling the Google app like you said, but then I just get a toast message saying "App not installed" whenever I try to search something from the homescreen widget.
Would appreciate if you have any tips on how to set Firefox as my search app!