https://addons.mozilla.org/en-US/firefox/addon/xmarks-sync/versions/ The Xmarks add-on at AMO isn't compatible with Firefox 57. Check with Xmarks support to find when they will have a Firefox 57 compatible extension available for their users.

XMarks is Not compatible with Firefox 57 Quantum. Bad News.... https://www.howtogeek.com/333230/why-firefox-had-to-kill-your-favorite-extension/ Nothing in this : https://mozilla.github.io/extension-finder/ Will have to google for a alternative and https://alternativeto.net/software/xmarks/ Several including Sync which is in Firefox. 3 Bar Menu (far right) Click, is on Top. Fyi: https://www.howtogeek.com/333110/how-to-customize-firefox-quantum-and-remove-the-white-space-around-the-title-bar/ Please let us know if this solved your issue or if need further assistance.

Thank you.