Unrecoverable error: [file]cannot be written!

[therube]

Create a new directory, say, C:\DOWNLOADS.
Set FF to save your files into C:\DOWNLOADS.

See if that works for you?

[niltwill]

OK, if you have this problem...Then that specific folder you chose is probably lacking permission with users. Here is how I fixed this: Right-click on the faulty folder, go to Properties (Windows Explorer). Now go to the "Security" tab, then click on the Edit button under "To change permissions, click Edit". In that window, select "Users" and tick the pipe next to "Full control", then OK. Better yet, do this on the drive itself (in my case, D:) so this issue disappears (this process can take a while though).