Yeah, I'm having the same problem. It might be related to the fact that Ctrl + ] is also the keyboard shortcut to nagivate back in firefox [1], so when I try it I end up going back to the previous page. Ctrl + [ does nothing.
Or it might be that in my keyboard layout, [ and ] requires pressing RIGHT ALT + 8 and RIGHT ALT + 9, respectively.
Or it might be that in my keyboard layout, [ and ] requires pressing RIGHT ALT + 8 and RIGHT ALT + 9, respectively.
[1] https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perf...
PS: Indeed changing the layout to en-US fixes the problem, but that not a real solution.