Firefox previous/next shortcut

Hello,
The firefox shortcut for previous/next is Cmd+[ and Cmd+]

On a German keyboard of the late 08 MB, to type the "]" or the "[" sign, I have to press Option+Command+5 or Option+Command+6

( What is btw very strange that the sign is not even visible on the keyboard and I had to look for it )

The problem is that if I hit this key combination in Firefox, it doesn't trigger the previous/next action. The "History" menu is highlighted for a short moment as if the combination was correctly type but the navigation doesn't move.

An alternative would be to use cmd+left/right but this actually doesn't work when an input field of the page has the focus ...

Any ideas how to do that ?

Thanks,
Tex