is there a button or something like that to delete all Junk mails at once?
I sort the mail atm and select it with CTRL+SHIFT+UP and than I press delete.
In your case, it's
deleteJunk
Get the Keyconfig extension
Following the guide in the above link, create a new shortcut key for the desired command
(Copied from above link)
goDoCommand("cmd_compactFolder")
As of April 2017, Thunderbird has a menu item: Tools > Delete Mail Marked as Junk in Folder
That does not speed up the process much though, say if you have several email accounts each with their own junk folder, it would be nice to have a single button to just empty them all.
Such as the Empty 'em extension I found on the addons site https://bluebox.live/emptyem
This allows you to empty trash and junk for all servers in one go, even has options to compact the folders (to truly remove everything) and suppress all the confirmation messages, so you don't, like in my case, get 20 confirmation boxes asking you if it's ok to delete the junk (yes I have a few email accounts lol).
Took less than 3 minutes to empty and compact them all with just 1 click though.
Answering here on this old thread because this was the first result when I Googled today, five years later, and the best answer here is no longer valid (see below).
"Mad Man With a a Blue Box" - you are a godsend.
Only downside is that the extension you mentioned ("EmptyEm") is no longer available.
HOWEVER, because of that, I was able to find this extension (Xpunge) that does the same thing and works super well - just remember to go into the Add-on settings and select the folders you want to use before trying it and wondering why nothing was happening (like I did).