As with other software where multiple versions are in active use, I don't see we'll need a special handling here. IMHO there are different groups of folks around in this context:
- those who upgraded to FF57 and never used any addons are not affected.
- those who upgraded to FF57 unaware and now suddenly find many of their addons broken will ask for "alternatives to X" anyway.
- those who, aware of the changes, did NOT upgrade, or switched to a fork like Pale Moon (remaining in the "old world" of XUL/XPCom without WE support) or Firefox ESR (currently at v52 feature-wise) will find those existing answers still useful
- those having switched to Waterfox or Basilisk (compatible with both worlds, XUL/XPCom as well as WE) will find both types of answers helpful
- those who switched to an entirely different browser altogether (Vivaldi, Otter-Browser, qutebrowser) are not affected anyway either way
The only thing we should avoid is closing new questions clearly targeted at FF57+ as duplicates to those targeting older versions.
(Note that all alternative browsers/forks I've mentioned are cross-platform, so most Firefox users are covered)