OK, I have a workaround but I'm embarrassed to say how much time I spent trying to get it to work.
There are 2 parts of it
1. Periodically i need to manually export Chrome bookmarks to a file from the Chrome bookmarks manager (Ctrl-Shift-O).
2. Configure the FarrFox plugin so that Search Firefox Bookmarks is checked and the path to the exported bookmark file is listed under Custom bookmark file locations (without quotes).
It still shows the Firefox icon though. Even if I extract the Chrome icon from Chrome, as soon as I rename the file to Firefox.ico to try and fool the plugin, the icon changes to a Firefox icon in Windows Explorer! Just from a rename! And FarrFox still shows the Firefox icon. But I'll live with it, this is close enough. EDIT:
Well, I just tried it again and it shows the Chrome icon now. Weird.