Thunderbird had automatically downloaded yesterday in my computer its latest version 45.3.0.
Imagining that that downloading might have introduced the Junk mail problem, after posting my previous query I newly downloaded the same latest version from Thunderbird's web site.
But the problem continues to be there.
Until yesterday, my Thunderbird email client used to nicely automatically send to my Junk box any email appearing in my Inbox from whoever which I had marked in the past as "Junk."
The above facility has mysteriously stopped today so that the emails in my Inbox with the "Junk" marking next to them continue to stay in the Inbox. (I have to then manually transfer those Junk emails to "Trash.")
How do I restore the facility which I mentioned above in the first paragraph?