This seems to be somewhat badly written copy on Mozilla's part. To clear up what this change actually means for an end user:
- You can still manually install extensions. From now on, all installations will need explicit user confirmation.
- No extensions can be installed silently. This is what sideloading did, all extensions in a special folder were installed in all Firefox instances on the computer without the user's consent.
This is most definitely a Good Thing, as it means for example no malicious extensions can be silently installed by malware etc. Communicating this change could've been done better, though.
- You can still manually install extensions. From now on, all installations will need explicit user confirmation.
- No extensions can be installed silently. This is what sideloading did, all extensions in a special folder were installed in all Firefox instances on the computer without the user's consent.
This is most definitely a Good Thing, as it means for example no malicious extensions can be silently installed by malware etc. Communicating this change could've been done better, though.