Oops, this is embarrassing... I just made the same mistake again as from the other thread that I linked to.
That's a nuance with terminal commands, casing matters. The correct command is with an uppercase "T":
Sorry about that... follow the instructions I gave in the post above: delete the symbolic link you just created and run the same command, but make sure you enter capital "T" Thunderbird:
ln -s ~/.var/app/org.mozilla.Thunderbird/.thunderbird ~/.thunderbird
Yes, any file that starts with a dot (like in .thunderbird or .var) is a "hidden" file and won't show up by default. You can toggle this setting in Nautilus through the preferences menu. There are quite a lot this which is why it's disabled by default.
Well, sometimes 