Open the library, right click and you get an option to create a new folder; or use the Organize menu from the top of the library. Similarly you can right click within the dropdown lists or sidebar list.

• see sorting bookmarks <-- clickable link --
• and how do i use bookmarks?
• you may also be interested in what are tab groups? if you move to a newer version of Firefox
  (You appear to be using Firefox 3.6 ; Firefox 6 is the current release version.)

Is your problem merely the number of bookmarks or folders visible on a screen without scrolling (typically <<50 )

I am not sure what the limits are, guessing I would say probably somewhere in the order of 10s of thousands of bookmarks. (In some extreme instances numbers of bookmarks may impact on amount of history entries that may be retained)

You can create new folders, and nest them as folders within folders. The tabs toolbar for instance can contain nested folders. Firefox can have problems with moving around large chunks of bookmark folders sometimes, in which case the task may need breaking down so that smaller groups are moved.

There shouldn't be a limit to the amount of bookmarks in a folder.

A possible cause is a problem with the file places.sqlite that stores the bookmarks and the history.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes