@bastyani
FF can NOT use certificates imported in the system. You seem to have some issue with your tracker updating. That is why you don't see new txt files in your Documents folder. If you resolve issue of document files not displayed in Document app, than you will be able without any problems to import certificate to FF and use it.
Try rebooting your phone.

PS. Do you have issues with files when populating Gallery app?
If you move image to /home/user/MyDocs/DCIM ot Pictures - does it show in Gallery app?