Can you check to see whether your mailto setting got changed?

orange Firefox button or classic Tools menu > Options > Applications

In the Search box above the list, type or paste mailto and pause while Firefox filters the list. On the right side, you can select your mail application. Unless it's missing. Are you able to establish the connection?

I checked it before asking the help. It didn't change.But content type "mailto" looks strange. It lost the icon.

You can look at the network.protocol-handler.external.mailto prefs on the about:config page.
If network.protocol-handler.external.mailto prefs are user set (bold) then right-click and Reset them to the default value.

• http://kb.mozillazine.org/about:config

Fantastic! It starts to work. Thank you!

You're welcome