SpellCheck: F7 does not work in every browser

problem:

• Usage of F7 to start the SpellCheck (eg instead of using the SpellCheck-Button in the statusbar) does not work in every browser.
  Examples (Ubuntu):
  Google Chrome: OK
  Firefox: NO (https://www-archive.mozilla.org/docs/end-user/moz_shortcuts.html)

solution:

• Find another shortcut (eg F7 with CTRL, SHIFT, ALT).
  Caution: Do NOT try CTRL+ALT+F7 in Ubuntu!