You can enable "Emacs bindings" in a bunch of applications. Enabling that in GTK would let you use the cursor movement keys in Chrome and Firefox.
I use a variant of the emacs key theme, so I'd be happy if there were a workaround...
It works in Chrome, but unfortunately not in the standalone Slack client. (Maybe an argument for switching to the web version?)
(Uh, does anyone know how I select all (previously C-a) or search (previously C-f) in Chrome now??)
EDIT: Looks like C-/ functions as select all, still looking for a find replacement.
EDIT #2: Oh, silly me, as long as I'm not focused on an input, C-f still works.