From the Bookmarks menu of the menu bar, select the Organize Bookmarks option. When that window opens, from its Import and Backup menu, select the Export to HTML option. A standard Save As dialog comes up to let you choose what to call the file and where to save it. After it's created, copy that HTML file you just created to the other PC. Repeat the same procedure of choosing Organize from the Bookmarks menu in Firefox on the other PC, etc., but choose the Import from HTML option instead of Export from the Import and Backup menu. Make sure the From File radio button is checked in the first dialog of the Import wizard--it should be by default--then just follow the prompts to select the HTML file you just copied over, etc., to import the bookmarks.
Good luck, and hope this helps!
