Xmarks was bought by LastPass! and then extension in FireFox was updated to v4.01. Since then, I have had a problem when opening a new window, that the address bar just didn’t work, bookmarks didn’t work. Luckily I had Xmarks on my laptop, and it was the previous version of it, 3.90.10. Copied it over and problem vanished. Then, just for giggles, I replaced v3 with v4 again, and problem reappeared immediately. So, put v3 back and all is well again.
I’m betting their tech/help will still say it’s something I’m doing wrong.