Maybe one day if someone wants to implement a new application to do this (would require extra installation on your system) per Native helper application · Issue #23 · kee-org/browser-addon · GitHub . It’s not something I’m planning to work on myself in the foreseeable future though and there is no guarantee it will actually work.
Yes, the new addon architecture is what has caused this feature to go away. With another year or two’s notice of the removal of the old system, we might have had time to investigate alternative possibilities such as #23 but it’s just way too much work to be possible in the time frame Mozilla provided.
This is the standard Firefox label style so you might want to offer your feedback to them.