I am constantly inadvertently changing the "Fixed default download folder".

a couple suggestions that would keep this from happening...

make a change so that, when FDM opens and the "New Download" dialog comes up,
the focus (cursor) is not on the "Save to" box but rather the "File Name" box.

or

Under Preferences > General > Default Download Folder > Fixed default download folder
add an option to lock it so it cannot be changed.

Thanks