Firefox stores the Bookmarks inside a file named places.sqlite which is located in /Users/Username/Library/Application Support/Firefox/Profiles/someRandomName.default/ .
On machine one:
1. Search this file and make a copy of it and rename the original one to _places.sqlite (our backup file if something went wrong).
2. Now put the copy in your Dropbox folder, e.g. Dropbox/Firefox or some other folder you like.
3. Open the Terminal and create a symbolic link from the Dropbox “places.sqlite” file to user Firefox/Profiles folder “places.sqlite” file.
ln -s /Users/username/Dropbox/Firefox/places.sqlite /Users/username/Library/Application\ Support/Firefox/Profiles/randomName.default/places.sqlite
Note: you have to modify the path according to your filesystem names (username & randomName.default)
On all other machines that should sync with the Dropbox “places.aqlite” file repeat step 3 and modify the path.
I’ve also tested to move the whole Profiles Folder to the Dropbox and it worked. The result was a Firefox running on two machines with all the luxury of saved tabs/history/plug-ins and so on. The dark side was that i got “file updated” spam from Growl every few seconds. At the moment i’m fine with syncing the “places.sqlite” file only.
Check http://kb.mozillazine.org/Places.sqlite if you wanna know what is stored in places.sqlite.