Firefox add-ons extension not showing

Today I started to develop Firefox webextension but I have some problem.

I clicked "Load temporary add-on" button to install example webextension, and chose example webextension, It seemed that it was installed but it did not show in extension section.

To debug, it should appear in "Extensions" section.

My firefox version is 51 now, I already tested with 47, 48, 49, 50 but didn't work...

Anyone know?

Thanks.