Thanks for the report, but that has never been the case for me.
On Windows and Linux, Alt Gr + Q is used for @. Not Alt + Q. Alternatively, Ctrl + Alt + Q can be used, which also does not trigger the add-on.
I've researched Mac, but they seem to use a completely different combination for the @.

Could you give some insight into what system uses this shortcut for the @? (It's not the keyboard layout, as I'm using the german keyboard layout as well)

Sadly, there is no way to change this from firefox. chrome would allow to edit those shortcuts. But I could make this different per OS.
The only other way would be to use a different shortcut or to not have a shortcut. Let me know if you have suggestions.