Unable to install firefox add-ons

[Gianluca Sforna]

Am I the only one unable to install firefox add-ons with
firefox-1.5.0.10-5.fc6 ?
For example, trying to install this:

https://addons.mozilla.org/firefox/60/

I get: Software installation is currently disabled. Click Edit
Options... to enable it and try again.

Fact is, I never disabled it nor happen to find where I should
activate it in the options...