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Firefox captures Alt+Shift, but Alt+Shift+" is set as a dead key for my layout.
I do some typing in other languages, so I've set up a custom keyboard layout with all the accented characters. I've set Alt+Shift+" as the dead key for the diaeresis (e.g., alt+shit+" i gives ï). They layout works system-wide, and I can use my other dead keys in Firefox, but not that one. It's not absolutely necessary, but it's pretty annoying.
Alle antwurden (1)
Some keyboard layouts like the "US International keyboard layout" can use the double quote (") as a dead key to add a diaeresis (I have a compose key on Linux), maybe that works better.