Under the bookmarks menu item, when I scroll down to the first submenu item it won't move down to any others.
I click on the bookmarks menu which has several sub-menus. If I want to go to, say, the third sub-menu, I have to scroll right to it. If I stop, even briefly, at the first or second my cursor stops and will not move further down (or back up). I have to exit the menu and try again.
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It is possible that you have one or more bookmarks with a long title.
The width of a column in the Bookmarks menu list is in current Firefox versions automatically adjusted to fit the bookmark with the longest name.
You can check the names of the bookmarks and make the longer ones shorter.