Hi, I'm not familiar with Mac but in the absence of other replies... I see from your screenshot that you have an Apple and Firefox button to the left of 'File' on your menu bar. I'm not sure why they are there, but they're moving the toolbar items - including Bookmarks, to the right, and that is likely to be why your folders open on the left.

I use the bookmarks button, which I've moved to the left-hand end of the bookmarks toolbar, and all folders open on the right - they have to! (You can move it by opening the 3-bar menu > Customize, and dragging it, > Exit Customize to save).

Hope that is of some help.

This can possibly happen because of the length of names in the first panel that leaves not enough space to open the folders to the right, so Firefox switches and open the folder to the left.

That can be caused by having some bookmarks with a long title in the main Bookmarks menu. The width of a column in the Bookmarks menu list is in current Firefox versions automatically adjusted to fit the bookmark with the longest name. You can check the names of the bookmarks and make the longer ones shorter.

I customized the toolbar so I could use the icon for the bookmarks as first suggested. Works well for me now. I hadn't used it before as I found the location over to the right less convenient. I also checked for overly long bookmark names - turned out I had one in the unsorted bookmarks at the bottom of the list. Shortening it made the bookmark folders open to the right again. Thanks for the help!