This might vary from one web browser to another, but I've just taken a look in Pale Moon (which is my normal browser) and it's done by:
-> Bookmarks menu -> Organise bookmarks (or just Ctrl-Shift-O)
Then, right-click on the parent folder to open a menu, and from that click Sort by Name at the bottom.
I don't actually organise my bookmarks at all because of the way I use them*, but to check this works I created three folders in Unsorted Bookmarks (One, Two, and Three), and after doing the above (as well as all the unsorted bookmarks now being sorted alphabetically, those folders are now One, Three, Two - i.e. they're alphabetical as well.
* I disable use of history when typing into the URL bar, limiting it instead to bookmarks. So I can just start typing something meaningful there, and the more I type, the more it's narrowed down to the right entry in my bookmarks.
Edit: Ah. Okay. :)
-- Edited by VinceH on Sunday 25th of August 2019 11:44:44 AM
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Vince M Hudd - Soft Rock Software
(I only came here looking for fellow apiarists...)