Joe, you can certainly partially clone a profile by copying a lot of data from one profile into a new profile. But the data will then diverge as you use each one independently. If you could live with that, then you could go to the about:profiles page and create a new profile. Then you would press the "Open Folder" buttons under the current and new profiles, fully close Firefox, and copy files from the original into the new folder. For a list of files to potentially copy see this article: https://support.mozilla.org/en-US/kb/about-your-important-data-and-their-files ---- I mention the above for completeness. I am guessing from the title that what you actually want is continued mirroring of new data between the profiles. That I can't really help with unfortunately. I would suspect that it might be possible if you could setup Firefox Sync with the same Mozilla account on both the original and newly created profiles, but not sure if that is possible or may lead to other problems. ---- Another possibility comes to mind but it would only work to mirror specific types of data. If you created two profiles then replaced the original "places.sqlite" and "favicons.sqlite" in one with symlinks that point to those same files within the other, then you would theoretically have two profiles that mirror each other's bookmarks and history. This could be repeated for password database files and others, while allowing independent UserChrome.css files. But I doubt that this approach is either sensible or issue-free. Again, mentioned for completeness but I would not recommend it. ---- Sorry that I can't be more help. If someone else knows another method for achieving your aims then hopefully they will see this and post. But I have a feeling that it will either be two profiles plus Sync, or one profile plus the method that you have already mentioned. For example, creating two shortcuts on the desktop that point to scripts or commands that literally just rename the UserChrome.css files and then launch Firefox.