Could you describe this in more detail?

Which menu:

• classic Bookmarks Menu on the menu bar
• Bookmarks Menu button (next to the star icon on the main toolbar)

Which method are you using to scroll the menu:

• holding the mouse pointer over the tiny triangle at the top or bottom of the menu
• using the mouse scroll wheel
• another method (?)

classic Bookmarks Menu, mouse wheel.

Okay, I think I see what you mean.

When the mouse pointer is not over a folder, and I scroll the wheel without moving the mouse at all, the pointer stays stationary on the screen and folders do not open when the mouse pointer rolls over them. So the problem does not occur.

However, if I move the mouse pointer by moving the mouse, an individual bookmark or folder is highlighted, and the individual folders expand to a submenu. When the mouse pointer ends up over a folder on the menu, the scroll wheel is ignored. In order to scroll using the wheel, I have to move the pointer off of that folder over a regular bookmark.

I have no idea why it works that way. Maybe the "listener" for scroll wheel activity moves away from the main menu to the submenu. ??

I haven't checked whether anyone has filed an official bug report for this, but probably someone should.

Personally, I have stopped scrolling the menu and use a custom style rule to split the menu into columns, but I don't know whether you would like working that way. https://userstyles.org/styles/119797/bookmarks-menu-in-multiple-columns

Yeah, that's exactly what i'm talking about. But the columns style is not the solution for me.

You can file a new bug here:

https://bugzilla.mozilla.org/enter_bug.cgi

As you enter your description, the site will check for duplicate bugs, but it's not perfect, so possibly you won't see an earlier-filed bug. Not a big deal, it's just an issue of making sure they really understand what's going on.

As a data point, I downloaded the "Portable" version of Firefox 38.7.1 (extended support release version) and it did not have this issue. So it started at some point after Firefox 38.

If you want to try to narrow it down further, you could try other "portable" versions -- they install into a separate folder and don't interfere with your regular Firefox installation. Of course, they also don't contain any bookmarks, so after you start a version, the first step will need to be to import your bookmarks for testing.

The downloads are available here:

https://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox%2C%20Portable%20Ed./

The stupid layout blocks the most important parts of the name of files and folders, but you can expand it using a little script hack which is faster than hovering your mouse. Open the web console in the lower part of the tab using Ctrl+Shift+k, then paste this in the line at the bottom and press Enter to run it:

document.getElementById("files").style.width="100%";