1. First thing to do is head over to Dropbox, get an account, and install it on all the computers you’ll be syncing across. This is very straight forward and they have installation options for Windows, Mac, and Linux.
2. Pick which computer has your “master” profile (the one that will be the starting profile replicated across all your machines) and start there.
3. Close Firefox if it’s open.
4. Next, you need to find your Firefox profile folder. Details for this on all major platforms can be found here.
5. Once you have found your profile folder to sync, I STRONGLY recommend that you backup this folder somewhere.
6. Find your Dropbox folder (defaults to /Users/[username]/Dropbox on a Mac and C:\Users\[username]\Documents\My DropBox on Windows), create a new directory to store your Firefox profile in (I used firefox-profile).
7. Copy the contents of your Firefox profile folder (found in step 4) into the newly created Dropbox folder created in the previous step. It should start syncing immediately, I recommend waiting till it’s complete before continuing.
8. Store the name of the Firefox profile folder somewhere, and delete this folder.
9. Now we need to create a symbolic link between the Firefox profile folder (which you backed up, right?) and the Dropbox folder we just found in the last step. On Mac/Linux the command for a symlink uses the syntax ln -s target link_name – which you may need to run as root (On Windows, the command uses the syntax MKLINK /D link_name target – this should be run from the command prompt and you may need to run this as admin in order to complete this. You can run this as an admin by going to start, typing cmd, right click the command prompt option and choose “run as administrator”) where link_name is the original Firefox profile folder (found in step 4) and target should be the shared directory in Dropbox used to store the profile (I used firefox-profile in my Dropbox folder – be sure to use the full paths for simplicity).
10. Repeat steps 8 & 9 for each computer, finding the Dropbox and Firefox profile folders on each system. Also, be sure to close Firefox completely before modifying the profile folder – and back up if you’d like to keep that profile info.

And that’s it! When you reopen Firefox it should open using the shared Dropbox directory and sync all your plugins, bookmarks, session history, etc across all of the machines. There are a few things I should mention as well though. First, you’ll probably want to turn off Dropbox notifications, as these files change often while browsing and will be resynchronized with each update. Notifications for this can get annoying quickly. You also might notice some weirdness if you browse on multiple computers at the same time…both keep trying to save their session history (and everything else) to each other and will continuously overwrite each other. Shouldn’t do any permanent damage, but worth thinking about.

Let me know if you have any other suggestions of if you get this working for yourself in the comments below.