Javascript isn't the hard part here; it's trying to figure out how to extract anything like what I need from the places.sqlite databases that hold all the relevant data.
If you want simple and dirty.
1. Create a backup of your bookmarks using the import and backup button on the Library window on each machine
2. Pick your favorite scripting language
3. merge the two json backup files using your script
Import the resulting json file in a clean Firefox profile.
I've tried this. The tags are not in the JSON dumps. The Mozilla people apparently believe that backing up your bookmarks means to back up everything except the tags.
A corollary: The only way to actually back up your bookmarks is to back up the places.sqlite file. Nothing else contains all the relevant data.