While using firefox sometimes we are unable to use the installed addons. In this case kindly follow the following steps to resolve the issue.
Step 1: Type about:config into the address bar and press enter. (accept the warning)
Step 2 : Search for "xpinstall.signatures.required"
Step 3 : double click on it to change 'Value' from True to False.
Step 4: Restart the firefox
That's it
