Accidentially I hide the menu bar in Thunar file browser.
How can I re-show/unhide it?
How can I hide the icons left of each files/folder?
You can restore the menu bar by pressing CTRL+M.
I think this option should be moved to Preferences menu, because it's a common mistake to accidently uncheck "Menubar".
As for the second question, I have no idea... (I'm not sure I understand it though)
I think pstein is refering to the Tree folder, if that is what you are refering to, it can be hidden/shown with F9.
No I mean the icons on the right side. See the following snapshot:
Furthermore I would appreciate to reduce the line height/space gap between file lines
So again: How can I achieve this?
You can change the spacing between lines in the right-hand pane with the View/Zoom in, Zoom out options, or with Ctrl +-
Are you sure that you want to remove the icons? That would mean you could no longer tell the difference between a folder and a file.