Please see if this article helps:

http://support.mozilla.com/en-US/kb/Lost%20Bookmarks#w_no-bookmarks-shown-in-the-bookmarks-menu-or-toolbar-after-installing-an-add-on

Thank you for the information, but unfortunately this didn't fix my problem. FYI: my bookmarks still appear when I click the Bookmarks from my menu; I just want them to appear in the left-hand column as before. It is important that this is fixed or I'm afraid I will have to find a new browser that supports this feature.

OK, I see. Try: CTRL + B instead.

I have tried that too and the bookmarks menu will open on the left but it is still completely blank. I've even tried reinstalling FF4 with no luck regarding this issue.

My last suggestion will be to backup your bookmarks (Show Bookmarks > Import & Export). Then try to import it again to see if that helps. Be aware you might have duplicates if you do so.

If it still doesn't work, someone else will have to step in and provide further help. Sorry.