There are lots of solutions for syncing Firefox, and I must confess I haven't used any of them. As I've read the descriptions, I've always gotten the sense that they wouldn't do what I wanted, the way I wanted it, when I wanted it. Necessity being the mother of invention, I wrote my own shell script to do what I needed.

I have Firefox running on four Linux systems and one Windows system, and all of them have the same extensions installed. My sync is a simple rsync of the Firefox settings directory from another system. This copies bookmarks and passwords, and I have used it for quite some time without any ill effects:

#!/bin/bash

user_name=mitch

if [[ ! "$1" ]]; then
	echo "Usage: $0 FROM-HOST"
	exit 1
fi

# Check for cygwin.
if [[ -d /cygdrive/c ]]; then
	rpath=/home/$user_name/.mozilla/firefox/default.enr
	lpath="$(find /cygdrive/c/Documents\ and\ Settings/$user_name/Application\ Data/Mozilla/Firefox/Profiles/ -iname '*.default')"
else
	rpath=~/.mozilla
	lpath=~/.mozilla
fi

rsync -av --delete --exclude=Cache --exclude=lock "$1":"$rpath/" "$lpath"

If you have Cygwin installed, the script will work from Windows systems also. Its usage is simple: first shut down Firefox, then invoke the script and give it a hostname or IP address to sync from:

$ syncff my-other-system

Obviously, you will need to modify the script to make sure that the paths it uses are valid for your installation.

One shortcoming is that this script will not work with a Cygwin system as the other system. That should be possible, but the script will need modifications and testing to accomplish it. When syncing Linux systems, I sync the entire .mozilla directory, which should probably work even if you use Seamonkey or have the Eclipse browser component installed, but test it to be sure, and shut them down before syncing also.

You may notice the rsync command excludes not only the Cache directory but a file named lock. If you don't exclude the lock file, Firefox will think that another instance is already running when it starts up.