Up to now, I think new bookmarks are added at the end of the list, unless you move them to a folder. What is different on yours? Or how did you sort them in a different way?

Sorry, my question was poorly worded. I know new bookmarks are added at the end of the list, and I sort them alphabeticaly in ascending order on my PC. But, up to now, when the SYNC function of firefox was executed on my PC (manually or automatically), the bookmarks were sorted on my smartphone Android too. My smartphone is connected to my PC Firefox. Similarly, any modification made on my PC was reflected on my smartphone. I think it's since the last update of firefox that the SYNC function doesn't work anymore.

I've never thought about how Desktop Bookmarks are sorted when displayed on Firefox for Android. As a test:

(1) In Firefox on Windows, I sorted one subfolder of my Bookmarks Menu and changed the order of two folders

(2) I launched about:sync, a page added by the About Sync extension (https://addons.mozilla.org/firefox/addon/about-sync/) that lets you view what is in the Sync cloud. Under Bookmarks > Client Records, which is a flattened list of the bookmarks for the current Firefox installation, I could see that the changes had reached the server.

(3) Checked Firefox Nightly for Android. Changes hadn't hit yet. I used Sync Now to force an update. Now I saw those changes in the Desktop Bookmarks > Bookmarks Menu folder.

So I think as long as there has been time for the changes to move to mobile, they should still go. That said, it's always possible for some corruption to creep in somewhere that breaks the process.