This end-of-support is obviously to correlate with the end-of-life of Firefox 52ESR, the last version to support legacy add-ons. That's logical, but it sucks, particularly for people who are stuck on 52ESR (Windows XP and Vista come to mind). Naturally, this also sucks for alternative branches such as Waterfox which split off before WebExtensions became mandatory, and the poor beleaguered remnants of SeaMonkey.
For TenFourFox users, there is an archive available from SourceForge of the last Firefox 45-compatible versions of popular addons, both classic and current. Naturally this archive is not comprehensive and won't ever be, though I'll consider adding other addons I believe are notable. Download and drop the XPI on any open browser window. Other users are welcome to grab stuff from our archives, but I consider 52-compatible versions out of scope, so please don't ask.
For OverbiteFF users, I will probably move it back over to Floodgap's gopher clients page and maintain it there, since if you have an interest in Gopher I can pretty accurately predict you probably don't use a vanilla stock web browser either. However, because OverbiteWX and OverbiteNX are fully compatible with WebExtensions, they will not be affected by this change.