I am also looking forward very much to a Xmarks Webextension-compatible version.
With Firefox 54 onward, development has geared towards multi-processing, if compatible add-ons allow to use this, otherwise multi-processing is disabled.
The current version of Xmarks still runs in Firefox, but is also disables multi-processing, which is a shame as this does not increase the speed and stabilty of the browser.
As far as I understand, in Firefox 57 (due november 2017) all incompatible extenxions will automatically disabled, and the nightly of 57 already shows that this is true.
So please LastPass & Xmarks, start to into develop a compatible WebExtension for Firefox!