If you installed the Flatpak version of Thunderbird, that may have something to do with it. Long story short, Flatpak is one of various package formats that exist in Linux, and it has restricted access to the rest of the filesystem by default.
You can verify this easily by running the following in the command line:
flatpak list
If you see it listed there, the easiest way to fix it without having to change anything in your settings is to install another package called Flatseal. Take a look at this comment for reference:
You'd have to pick the right location the NAS is mounted on your filesystem and allow Thunderbird permissions to that.