text file is not display in firefox browser, instead it keep asking to be open in an external program ie. notepad
Firefox should be able to open a text file by default, mine does.
Maybe you (or an extension) have added a specific action for text files that is no longer possible so it is defaulting to asking or offering to download.
Check any actions listed under file type .txt make a note of what it is currently then delete it.
That happens if the text file isn't send as text/plain, but with another MIME type.