There 5 ways to save a new Bookmark in Firefox. All except for the single-click on the star allow you to select the folder for each new bookmark that you save.
1. {Ctrl + D}
2. Right-click and select Bookmark this Page.
3. Bookmarks > Bookmark this Page
4. Double-click the Star that is on the right side of the Location bar.
4a. (A single-click on the Star will save the new Bookmark to the Unsorted Bookmarks folder.)
5. Left-click and drag the website image (Favicon) in the URL bar (or on the active tab) into the Sidebar view of Bookmarks {Ctrl + B}
or to the Bookmarks menu bar item (the drop-down will open) and then onto the drop-down listing of your bookmarks and folder of bookmarks (folders will open)
or onto the Bookmarks Toolbar - the new bookmark will be saved when you release the left mouse button

If you think that the Page Bookmarked window is too small and you can't see enough of your folders to make a selection from, try this extension.
Add Bookmark Here ² - https://addons.mozilla.org/en-US/firefox/addon/3880

There are three "sweet spots" where you can drag that dialog window up/down to make it larger.

Thanks for the speedy reply. All of those work but none of them answer my original question. Let me just say, I'm not having a problem with making a new bookmark. It's just that all the above suggestions will result in a new bookmark being added to the Unsorted Bookmarks folder. After which, I will then need to manually re-sort them them, either at the end of the browsing session or straight away - either option being carried out from within the Unsorted Bookmarks folder itself.

I can see that maybe some would choose to have the sidebar open at all times, just in case the need arose to create a bookmark. However, this seems like a waste of 12-20% of screen space. If on the other hand, the suggestion is to open the sidebar every time one needs to create a bookmark, then close it down. It would appear that the less onerous of operations, would be to just carry on with an end of session resort. So then we're back to, maybe the menu in the Folder: drop-down is a redundant throwback.

I gather from the initial reply, I was not exact enough with my query.

Only the single-click method automatically places the new bookmark in the Unsorted Bookmarks folder (by design), every other method allows you to select the folder in one way or another, as I mentioned already.

The manner of saving a new bookmark is the same as when Firefox 3 was introduced in June of 2008, imperfect but usable. There are a good number of add-ons created to change the way certain parts aspects work, so if you're not happy find an add-on that fixes what you don't like about it. I recommended the one extension that fixes what I don't like about saving a new bookmark - the window for selection is too small and there's now way to drag it larger to show more folders, plus the folders don't stay open between "saves". I can see all 20 of my major folders using that add-on, without having to do anything except {Ctrl + D}.

If you are using the default theme there are two down arrows next to the Folder: name in the Page Bookmarked dialog window. The arrow on the right opens a larger window (same as when you select Choose) where you can select any folder in the Bookmarks Menu folder by opening that folder (where all the bookmark folders that your create reside, outside of the Bookmarks Toolbar folder). Just select the folder you want from that larger window.

Personally in the Profile with my largest set of Bookmarks ( have 3 different sets of Bookmarks in separate Profiles), I open the sidebar using {Ctrl + B} with my left hand (little and index fingers), and drag the website icon with the mouse in my right hand, then close the sidebar with another use of {Ctrl + B}. With my widescreen monitor that "20%" isn't used too often for displaying page content, just a wide margin so sometimes I just leave the sidebar open.

Also, sometimes in a another Profile I drag the website icon to the Bookmarks button on the Navigation bar, which opens the Bookmarks dropdown and also the folder I want to place the new bookmark in and save a new bookmark in that manner. (I use different toolbar layouts in those 3 separate Profiles, which makes it easy to use different methods of saving a new bookmark in each, without getting confused.) I don't use the Bookmarks Toolbar in any Profile, I reduce the upper toolbar area down to only one or two toolbars to save space for page content.

"If you are using the default theme there are two down arrows next to the Folder: name in the Page Bookmarked dialog window. The arrow on the right opens a larger window (same as when you select Choose) where you can select any folder in the Bookmarks Menu folder by opening that folder (where all the bookmark folders that your create reside, outside of the Bookmarks Toolbar folder). Just select the folder you want from that larger window."

I noticed both of these arrows; if I select the larger of the two I just get a drop-down, listing Bookmarks Toolbar, Bookmarks Menu, Unsorted Bookmarks, with the option of New Folder. If I then select New Folder, only then am I presented with my existing bookmarks menu. From here I can then select a desired folder, but the new bookmark gets placed at the bottom the the list in the folder/sub-folder.

If I select the smaller arrow, I get Bookmarks Toolbar, Bookmarks Menu, Unsorted Bookmarks, with Choose. Now if I select Choose, I then get a drop-down of Bookmarks Toolbar, Bookmarks Menu, Unsorted Bookmarks, with the option of New Folder. And the above applies with regard to sorting.

"Personally in the Profile with my largest set of Bookmarks ( have 3 different sets of Bookmarks in separate Profiles), I open the sidebar using {Ctrl + B} with my left hand (little and index fingers), and drag the website icon with the mouse in my right hand, then close the sidebar with another use of {Ctrl + B}."

Using the Ctrl + B to open or close, is by far the easiest method and I'll use that in future. And dragging the website icon in the aforementioned manner does seem a valid method of adding a new bookmark. However, there is no way to edit the new bookmark's name before being placed in the folder. Though at least with this method, one can choose the exact position within the folder/sub-folder.

"Also, sometimes in a another Profile I drag the website icon to the Bookmarks button on the Navigation bar, which opens the Bookmarks dropdown and also the folder I want to place the new bookmark in and save a new bookmark in that .manner..."

Dragging the website icon in the aforementioned manner is also a valid method of adding a new bookmark. However, there is no way to edit the new bookmark's name before being placed in the folder. Though at least with this method, one can choose the exact position within the folder/sub-folder.

"... I don't use the Bookmarks Toolbar in any Profile, I reduce the upper toolbar area down to only one or two toolbars to save space for page content."

Me too.

As you say, it's not perfect but there are ways to pre-sort entries. Though if one likes short precise names for bookmarks, (and not the full website address and or the site's list of what can be found at said site) then a certain amount of editing and or shuffling will have to take place at a later time. I'll look around for an Add-on to help me and I thank you for your knowledge and help in this quest. I reckon for now at least, I'll have to mix & match different ways of adding new bookmarks and continue to edit newly added bookmarks at the end of each session.

Note. I've just come across the info that FF is a 32-bit application. Now I'm using it on a 64-bit PC. I checked beforehand to make sure I got the correct version. But the newly acquired information suggests the 64-bit versions of FF can be unstable. Maybe this is why I don't get the same range of menus as is suggested.