Thanks again to @amyt and colleagues.
Beta 4.2.0a was released on 2017-10-12 and is:
- compatible with e.g. Firefox 56.0.1
- not compatible with Firefox 57.
Another beta, announced on the same day:
- has a version number that’s inferior (220.127.116.11beta at the time of writing)
- is described by LastPass as for Firefox 57
- is also compatible with Firefox 56.0.1:
(Ignore the 1970 date in that screenshot. It’s a cosmetic bug, unrelated to LastPass.)
– but I do not intend to test with any 52.x extended support release (ESR).