First, don't panic.

I'm guessing that were not using Firefox Sync, or else signing in to your Mozilla Account in your new profile should have trickled in your data from your old profile.

Next Steps

Open the About Profiles page by typing or pasting about:profiles in the address bar and pressing Enter. The currently active profile that is not yet customized will have this message:

This is the profile in use and it cannot be deleted.

One of the other profiles is the one you have been using until the downgrade. Here's how you can find it, working methodically through all the possible candidates on the page:

(1) Click the Open Folder button on the Root Directory line to launch a Windows File Explorer window

(2) In the profile folder that opened, double-click into the bookmarkbackups folder

The files here are automatically created bookmark backup file in a compressed format. Each one should have the creation date coded into its file name. Does this folder have a recent file, such as

bookmarks-2024-11-25_249_gibberish=.jsonlz4

If so, this could be your recently used profile. If it only has much older files, it's probably not the one.

(3) Click up one level back to the main level of the profile folder and look for a file named compatibility.ini

Right-click > Rename the file to compatibilityOLD.ini

(4) Back on the open about:profiles page, try the "Launch profile in new browser" button for this profile.

Because you hid the compatibility.ini file from Firefox, it should not reject using an older version of Firefox with this profile, but in the case of actual incompatibility due to file format changes, some data might be lost.

Is this the one you want? If so, on the open about:profiles page, click the "Set as default profile" button for this profile. You're done.

If this isn't the one, repeat the above steps with the next profile on the page, etc.

Thank you so much! Yes, I was panicking. I did exactly as you said and everything came back good as new. Thanks again for the quick and knowledgeable reply.