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.

@TechHorse thanks, but you've taken the logic path I want avoided. Perhaps I was not clear. I want to run FF using just one profile, but able to direct it WHERE to find the UserChrome.CSS file. This is only about being able to test/debug my personalized UserChrome.CSS file.

Where does FF get the info that it should use a UserChrome.CSS file? I GUESS it is simple. It uses it if it is where it expects it to be. But, who wants to GUESS?

I would be looking for a Firefox runtime command line parameter, or a :CONFIG file parameter. I'll suggest it in the FF Forum.

Actually, I see now that just renaming .CSS files would not be very difficult, since .CSS code when it runs doesn't care if an INCLUDE file does not exist.

Hello

With the help of Browser Toolbox https://firefox-source-docs.mozilla.org/devtools-user/browser_toolbox/index.html

https://connect.mozilla.org/t5/ideas/option-to-disable-the-audio-icon-on-tabs/idc-p/105065/highlight/true#M55318

A illustration, test userChrome.css, in real time https://www.youtube.com/watch?v=9w_EVtYmN8s

@agent virtuel

   I have no idea what you are trying to show me with the links you provided.  The third, the youtube video, is particularly useless.   Instruction videos with no audio are absurd.   It's usualliy impossible to figure out what is being done or why or even how. l 

Can I use the browser toolkit to accomplish my goal? If so, you haven't told me how. It seems like the first two links are just directed to one's learning about CSS. I don't need education in adding CSS code to firefox and I don't understand why you suggested them.

the only thing I can GUESS is that there is some way to change the CSS code that is running WHILE using Firefox.

Interesting idea. What differences do want between the css files.

One idea is a script.

1. Two icons for firefox Fx and Fxx. Each icon is a script to work on the userChrome name and then open Firefox 2. Two userChrome.css (1 standard . . userChrome.css 2 the other userChromexx.css) 3. Each script will rename the css

Fx {script . / ../rn userChrome*css userChrome.css, run firefox Fxx {script rn userChrome*css userChromex.css, rn userChromexx.css userChrome.css

Something like that

Hello

Can I use the browser toolkit

In my video illustration test userChrome.css, i tested the code in real time.

With the help of https://firefox-source-docs.mozilla.org/devtools-user/browser_toolbox/index.html Style Editor https://firefox-source-docs.mozilla.org/devtools-user/style_editor/index.html

https://support.mozilla.org/en-US/questions/1520202 A other illustration, test userChrome.css, in real time, with the help of Style Editor https://www.youtube.com/watch?v=fLWg5KBtWQI