The preinstalled firefox theme with the latest installer ISO looks good, but is not usable for me in terms of overview with many tabs. It also can not open links from anywhere or drag and drop tabs to create a new window.
How can I reset it properly?
Remove the package
sudo pacman -R firefox-gnome-theme-maia
Restart your session, restart Firefox.
Did you look on the forum about this issue?
Yes I do – when typing the headline there are “similiar” topics shown. I thought “firefox” and “theme” will show the correct topics. And I normally scroll the latest topics.
Thank You anyways. 
What he meant was, did you use the “Search” function (magnifying glass in top bar) of the forum, before posting?