When I want to use FEBE to backup the Firefox extension, I got a message:
“You have 1 extension(s) pending installation that will not be included in the backup. …”

Then I searched for the solution. This happened because there existed “profile folder\extensions\staged-xpis”. And what I had done was just delete the “staged-xpis”, then the problem is solved. Because inside the “staged-xpis”, I found a “tmp.xpi”.