See if iCloud Bookmarks has updated or contact its support.

I called Apple to raise this concern. They insisted the blame is on Mozilla, for they need to "update their systems". I tried to tell them repeatedly that they need to update THEIR extension, but it wasn't easy talking to them.

It's your fault. No, it's your fault . . . . .

This is something you should think about.

Get another add-on that does the same job.

Downgrade Firefox to a version that works for you.

Two things: 1) how is it my fault that Apple doesn't update their software? 2) problem solved: back to google chrome

You're not my shrink

The extension should install, but recent comments show that there are serious issues with the extension in Firefox 55.

Note that there are still crashes reported related to an iCloud DLL (appleffdav.dll).

• Bug 1097138 - startup crash in appleffdav.dll
• https://crash-stats.mozilla.com/search/?product=Firefox&signature=%5Eappleffdav.dll&_facets=signature&_columns=date&_columns=signature&_columns=product&_columns=version&_columns=build_id&_columns=platform#facet-signature

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)

FredMcD said

It's your fault. No, it's your fault . . . . .

This is two children bickering. Sorry.