One convenient method is to export your bookmarks to an HTML file.

In the following article, please note that the menu items has changed from "Organize Bookmarks" to "Show All Boomarks".

Exporting bookmarks to an HTML file | How to | Firefox Help

If you have Ubuntu 11.04 then see this:

• /questions/822193

Ubuntu 11.04 displays window menus in the control bar at the top of the screen
Click on Bookmarks/Show All Bookmarks then move your mouse to the left upper corner toward Firefox Web Browser.
You'll get pop up menu (Organize, Views and Import and Backup).