I run dual 24-inch monitors here and like to have separate bookmarks for each monitor. I use FIREFOX here as well.
I run WIN 10 in WINDOWS 7 mode using a few icons down the left side of the screen and have screensavers going showing my night scenes.. Could never get use to seeing all of the small windows on the main screen with all the programs running in them...
I made up two folders in the bookmark locations named MONITOR 1 and MONITOR 2. Now when I add bookmarks I just drop them in the two folders.
One of the recent updates on WIN10 stopped me from auto loading the bookmarks at startup so now I just right click on bookmarks and click on the folders from each monitor. Then select load ALL bookmarks. I'm sure there is a way to do this automatically when restarting again - just never looked for it...
Roy Ken