Problems while typing special characters in Firefox on OSX

I just had the problem that I wanted to edit a wiki page and could not type some brackets because Option + 1-9 only switched the tab in my browser and did not result in a bracket as expected.

If you are experiencing this problem while having "SpeedDial" extension active simply walk to SpeedDial by clicking "Add-ons" in "Extra" menu. Then click "Settings" for this extension.

The third tab (In german named "Steuerungseinstellungen") provides a list of actions to do while pressing some keys. In there you can disable the shortcuts for "Alt + Number" by selecting "Do nothing" and your problem will be gone.