I am not sure on how to access the bookmarks bar directly using keyboard. However, you can access to the bookmarks bar through the Menu Bar using keyboard.

First, "Alt+B" will bring you to Bookmarks Tab of Menu Bar. Then, you can navigate to "Bookmarks Toolbar" and cycle through the bookmark folders using the arrow keys of keyboard.

You can set focus to the location bar and then press the Tab key until the Bookmarks Toolbar gets focus. Once the Bookmarks Toolbar has focus then you can use the cursor keys or press the first character of the label name to jump to the next occurrence.

See also:

• https://support.mozilla.org/en-US/kb/access-toolbar-functions-using-keyboard