There are a bunch of solutions on LQ and elsewhere. FF losing the file manager association is a bug | feature. Here is a quick and dirty trick. Find ~/.local/share/applications/mimeapps.list and edit to add or modify the line:
In this case, Thunar will be used, or if not found, then use Dolphin.
Normally you would use one of the mime update utilities, but I am too lazy to figure that out.
You can also try in "Edit - Preferences - Applications" to set "file" to "/usr/bin/Thunar" or whichever file manager you prefer.