[foepys]

At least uBlock Origin is compatible with multi-processes. I'm using it right now.

Firefox 55 marks classic extensions as legacy, this has nothing to do with multi-process comparability.

[nsm]

Yep, not legacy, but "not compatible with multiprocess" is what it should show for offending addons.

[inferiorhuman]

Well, how can one determine which addons are multi-process compatible?

[Vinnl]

https://addons.mozilla.org/en-US/firefox/addon/add-on-compat...

Then Ctrl+Shift+A.