Take a look at Waterfox [0] fork (at the moment based on Firefox 55).
Just successfully ported my ancient Firefox profile to it and all circa 20 legacy extensions are alive and kicking. Author seems to be willing to port features from Firefox trunk while preserving legacy features [1].
[0] https://www.waterfoxproject.org/ [1] https://www.reddit.com/r/waterfox/comments/79mwsc/waterfox_w...