Is it safe to sync a Firefox instance in both directions?

"I think you're asking to sync the contents of files such as bookmarks etc. in the purpose or merging data from the two FF instances. Just syncing files may get you in trouble.

For example, if a FF add-on keeps data in a file, and if one FF instance has a newer version of the add-on installed, you might end-up with the latest version together with the older data file, so nothing will work any more.

If you're looking to copy an entire instance from one computer to another, then you can copy entire directories from one machine to the other with no problem. You just have to know which directories. This may, however, land you in trouble if you ever decide to uninstall FF, since not all files are guaranteed to be cleaned out.

However, if you're looking at merging data, then John's answer is a good beginning for bookmarks. Syncing add-ons is best done manually. Syncing the awesome bar histories is possible but complicated."

"I'm using Dropbox to sync Firefox Portable across a couple of computers, and for the most part really like it and is definitely safe. That being said, there are a couple of problems that seem to pop up, but nothing that is a showstopper.

Problem #1 - If, for whatever reason, the two computers are not able to sync up firefox will create a new ""conflicted"" copy of the file.....something like urlclassifier3 ([computer-name] conflicted copy 2009-12-08).sqlite. This doesn't seem to cause many problems except to maybe lose some of your cookies to websites and take up extra space on the dropbox.

Problem #2 - There are some extensions that don't seem to be able to sync well this way. The two I use that I've noticed are ColorZilla and Chatzilla. They work on one of the synced computers, but not the other. I can reinstall the extension and get it working for a little while, but inevitably it reverts back to working only on one (a syncing problem perhaps?)"