-To download from pixiv you have to log in first (services>manage pixiv account)
-You can add boorus in services>manage boorus. Make a search on an unlisted booru, look at the address, search on listed boorus, compare the syntax, use common sense and try out different search syntaxes until you can download from the unlisted booru.
-You can right click the messages in the bottom right corner to dismiss them
-With the key shortcut ctrl+t you can open a window for a new tab and navigate this window with the arrow keys
-You can cycle through tabs with Ctrl-Tab and close one with ctrl+w like in any Web browser
-You can force idle mode in help>debug>force idle mode
These are things that took me some time to figure out.