Here is some more stuff to look at, in case you come back to your Mac before I do.

If the problem is that changes in Firefox are not being detected, click in the menu: BookMacster ▸ Manage Browser Add-Ons, and "Test" the Firefox Extension.

If nothing I've mentioned so far solves the problem, open your Bookmarkshelf document to the Settings ▸ Agents tab.  Verify that, at least, the first and third checkboxes, "Import Changes from Clients" and "Export Results to Clients" are swithced on.  Switch off the "Import Changes from Clients" checkbox, then switch it back on.  Doing that might fist it, because we've recently uncovered what appears to be a bug in Mac OS X wherein Agents just "stop working" until you either re-create them like that, or log out and back in, which causes Mac OS X to reload them.  I think we might have a fix for this in BookMacster 1.9.8 which we plan to publish tomorrow, but I'm not sure because the problem is sporadic and, although I've seen it happen myself,  we don't know how to reliably reproduce it.

Finally, if you want to send logs and other data so we can have a look at the problem, activate BookMacster and click in the menu: Help ▸ Trouble Zipper.

I have Bookmacster set up on two Mac computers. On each computer, I set up a single bookmarkshelf to sync Firefox and Safari clients via Dropbox. I'm also using iCloud to sync to iOS devices. Regardless of iCloud, my two desktop browsers are not syncing. No other syncing software is being employed (I used to use Firefox sync, but that was disabled prior to installing Bookmacster). Do I need to leave Bookmacster running in the background for it to work? The instructions don't say.