I just checked and assigning the shortcut in "System Settings>Keyboard>Shortcuts" will open Firefox and put it in focus.

For me in GNOME on Fedora this only works for chromium, for firefox I had to make an extension - I don't know why.

Is it flatpak or something or a normal install?

It's a normal RPM package.

Trade-offs abound, but this sounds amazing to me. I already have hotkeys to instantly switch between browser/terminal/Emacs/etc., so it's not an issue for me. What is an issue is some other application stealing focus while I'm typing elsewhere and (as another commenter mentioned) accidentally pressing space or some other key that performs an action in whatever pop-up or application stole the focus.

You should have focused more! /s /pun intended

But yeah, it has happened to me too, too often for it to be a problem. Especially the press the space bit.

I am not sure this is the right solution, however, but I cannot think of any solution right now.

Someone (in the comments) who has claimed to patch i3 may shed some light on what he did.