Rated 5 out of 5 stars
Finally I found out why I do not have Firefox's own "Find" button:
The button is available with the (ugly) standard skin of Firefox (Default 29.0.1) but not with the skin I use (Winestripe Realfox 4).
But good news:
I just installed the add-on "Find Button" (https://addons.mozilla.org/de/firefox/addon/find-button/).
And it works fine with "Winestripe Realfox 4" and "Findbar Tweak". :-))
Sorry for the long correspondence and
thanks again for your patience.