Your code have spacebars.

I tested this code and code is work:

set list_proc=firefox.exe thunderbird.exe filezilla.exe FileZilla Server.exe notepad++.exe AkelPad.exe AIMP.exe vlc.exe StrongDC.exe qip.exe
for %%a in (%list_proc%) do (taskkill /f /im %%a)

Good luck :)