In former version, users can install Firefox addon easily by drag and drop the addon file (.xpi file) into Firefox to install and use it.
But now, user can not do this by default. User can only install addon from AMO (https://addon.mozilla.org). If you download an addon from other website or like me develop addon self. when you drag and drop the file into Firefox. it will be disabled and can not be enable. It says : ADDON NAME could not be verified for use in Firefox and has been disabled. like this(I’m using Nightly Firefox):
well, it’s easy to pass this by these steps.
1: Navigate to about:config in Firefox (input about:config into Firefox’s address bar and hit Enter)
2: You might see a warning screen warning the action is not safe. Just hit the blue button I’ll be careful, I promise .
3:You will see a long list. Input the keywords xpinstall in the search bar.
4: Find the xpinstall.signatures.required, it’s value should be true now. click this line, change the value to false.
5: You are done, you will see that your addon enabled and working:)
Hank November 3rd, 2015