1) no & 2) no. One bookmarks collection per Firefox Profile; but you can have multiple Profiles.

https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Thank you for your reply. I will investigate profiles.

Since the bookmarks are exported as HTML, I don't understand why I can't divide the file. Do you know of a source of more information?

bookmarks.html is a legacy file in Firefox for quite awhile now, and not used for anything beyond being an interchange format for Import & Export of bookmarks among the various other web browsers out there. There is no way to have a bookmarks.html file become displayed in the user interface. Firefox hasn't used the HTML format for the user interface or for accessing bookmarks since Firefox 3.0 was released in June 2008.

But something that escaped my mind an hour ago is that by using File > Open File the bookmarks.html file can be displayed in the content area, like a "local" webpage. And the bookmarks.html file can be edited (but it uses a unique type of HTML code in places), along with being able to be "split" into different sections as long as this header is at the top of each section, iirc.

Example: file:///D:/bookmarks.html in a local file bookmark worked for me to "bookmark" a bookmarks.html file I have saved so I can access that file from other Firefox installations and Profiles that I have on my hard drive.

 <!DOCTYPE NETSCAPE-Bookmark-file-1>
<!-- This is an automatically generated file.
     It will be read and overwritten.
     DO NOT EDIT! -->
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=UTF-8">
<TITLE>Bookmarks</TITLE>

It has been like 10 years since I 'messed with' the !DOCTYPE NETSCAPE-Bookmark-file-1 file type so I am a little rusty.

IMO, just append the bookmarks.html file name to make each section have a unique name, so you can tell them apart and maybe store those sections / individual files in the same folder.

Thank you so much for your idea to use the bookmarks as an HTML file.

What you said also suggested another possibility to me. I can try using the HTML file to import the bookmarks into another browser, and export them from it to get them back for Firefox.

I never knew that the HTML file was not to be used for backing up bookmarks for Firefox. I came from Chrome to Firefox when Windows Vista (or maybe Windows ME) stopped supporting Chrome. I never had a problem with the bookmarks, thus never needed to use the HTML backups I made, and I never had a reason to see if I should change that decision, so I didn't know they were no longer supported. I just made the initial decision to select HTML among the two backup options because it was a human-readable format which I thought would be the safest if Firefox was discontinued (which seemed possible over its years).

So thank you again so much.