Try to run the code posted here in SQLite Manager.

• https://addons.mozilla.org/firefox/addon/sqlite-manager/

The last posted code (Query 4) will remove the descriptions. You can use query 3 to see the descriptions

• http://forums.mozillazine.org/viewtopic.php?p=12020177#p12020177

-- Query 3: test compound query
Select Content
From moz_items_annos
Where ID In (
Select Itm.ID
From moz_anno_attributes As Atr
Join moz_items_annos As Itm On Atr.ID = Itm.anno_attribute_id
Where Atr.Name = 'bookmarkProperties/description'
And Itm.Content Not In ('', 'Add bookmarks to this folder to see them displayed on the Bookmarks Toolbar')
)

-- Query 4: update Descriptions to empty string
Update moz_items_annos
Set Content = ''
Where ID In (
Select Itm.ID
From moz_anno_attributes As Atr
Join moz_items_annos As Itm On Atr.ID = Itm.anno_attribute_id
Where Atr.Name = 'bookmarkProperties/description'
And Itm.Content Not In ('', 'Add bookmarks to this folder to see them displayed on the Bookmarks Toolbar')
)

I was just testing and four of my 300 descriptions are for folders (presumably I added those to help myself remember the point of what was in those folders). If you have added descriptions to folders and didn't want to delete those, you would need something along the lines of the criterion I've bolded here:

SELECT moz_items_annos.content 
FROM moz_items_annos INNER JOIN moz_bookmarks ON 
  moz_items_annos.item_id=moz_bookmarks.id 
WHERE anno_attribute_id=3 AND moz_bookmarks.type=1

When you're comfortable that you're viewing just what you want to delete, then you could create a DELETE query. I didn't test that myself.

"Export" of bookmarks involves Firefox creating a bookmarks.html file, rather than doing a manual "backup" into a bookmarks.json format file. When it comes to doing an export, the descriptions for bookmarks is very small in comparison to the Favicon data that would exported. A bookmarks.html being exported can be as much as 25 times as large as the same bookmarks file from which the Favicon data was deleted.

@cor-el As I already had SQLite Manager installed, this worked like a charm. Thank you very much!

Is there no way to stop Firefox from adding the descriptions when bookmarking sites? If not, this would be an excellent addition.

@the-edmeister I already have favicons turned of inFfirefox through the about:config.

Again, thanks for the swift replies. I shall mark this resolved.