multiprocess was a different event and it happened well before Firefox 56 breaka...

pornel · on Sept 27, 2017

It wasn't enabled for users who had any old extensions installed.

noisem4ker · on Sept 27, 2017

"Legacy" addons could be made compatible with the newer multiprocess design. See: https://developer.mozilla.org/en-US/Add-ons/Working_with_mul...

Many developers went through the effort only to see support for their addons scrapped altogether a couple months later. Fortunately, that was quick enough for Mozilla to sweep under the "legacy extensions are unfixable and unmaintainable" rug.