Not sure if it works on a Mac, and probably only feasible on folders with minimal content and as an occasional workaround, but a method to make a permanent sort of a folder is:

1. Open required folder.
2. Sort content into required order using the context menu.
3. Copy and paste the whole folder.
4. The new folder is now permanently sorted and the original may be deleted.

it seems so basic that I can't figure out why Mozilla doesn't have the ability to sort all bookmarks. it used to, but that disappeared a longtime ago.

You can sort via right click, but only the folder you're in, one folder at a time. This can get tedious. .

Andy Halford's Sort Places handled this superbly, but he and Mozilla had a falling out. Fortunately I downloaded the xpi before he took it off his site, so I was able to install it on new instances of Firefox. However, starting with FF 22, it no longer works, and irregularly sends you error messages. Hopefully they'll make up and this and his equally excellent Check Places will return in an newer version

Auto Sort Bookmarks Add-on has a lot of complaints about its autosorting as soon as you install, and after you turn it off having to restore your bookmarks in order to get them where you want them.

Hopefully Andy will change his mind or Mozilla will put this very basic function into Firefox like it should be.