I have a lot of bookmarks - most of them are already sorted but some of them are not and I would like to sort the unsorted rest.

I think the simplest way for me to do that would be to somehow "enable" second sidebar with second listing of bookmarks - that way I could:
a) use one listing for opening list of unsorted bookmarks and click one one of them (to see what it was about)
b) use the other listing to drag-and-drop currently inspected bookmark into the appropriate group / subfolder
c) easy (without lots of scrolling) selection of the next unsorted bookmark on the list and repeat (a) + (b)

So, as in title, is there any way to show two sidebars, side by side, in Firefox Quantum?

As someone with a grand total of…nine bookmarks, I feel to have that many bookmarks is likely an edge case.

I almost feel it would be easier to export the bookmark file, edit it, and then import it after sorting.

Menu Bookmarks -> Show All Bookmarks maybe?

Yes that would be my suggestion as well, opens the bookmark manager where you can select multiple and drag them around and create folders etc., you could try having a second Firefox window open and drag from bookmarks to new window, in this way one Firefox window is used to show the webpage, whilst you organise the bookmarks.

Cntrl + shift + b
To open bookmark browser.

Are you sure you aren't that case? I have 1066 bookmarks for example.

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