Strange Firefox message box "550 Failed to change directory"

[Snurg]

I got this message box from FF a few hours ago I never saw before, and wondered because I am not aware of any ftp connection opened by me:

After I rebooted (because all memory was used up by zfs cache, possibly because I just copied the disk of by broken laptop onto the PC) and restarted FF, the box appeared again.
netstat showed me a ftp connection open:
tcp4 14 0 192.168.xx.xx.41749 annarchy.freedes.ftp CLOSE_WAIT

Any idea what is going on?

Could this be from a javascript running in a tab?
As there is a still embargoed bug in FF, listed on vuxml:

CVE-2018-5105: WebExtensions can save and execute files on local file system without user prompts

I wonder whether this could be related?

 

[Maxnix]

Did you try restarting Firefox in safe-mode (so all extensions are disabled)? Just to check if an extension is responsible for this.