If you wish to copy bookmarks from one machine to another, please follow the steps in this article: Backing up and restoring bookmarks.

Looks like what you actually want is to keep your bookmarks between Mac and Windows in sync all the time. To avoid this manual backup and restore steps, there are several add-ons that you can use. These add-ons will automatically make all your bookmarks available on all the machines without you having to manually copy them. For example, Firefox Sync and Xmarks.

Thanks so much --- I loaded Firefox Sync and it worked perfectly.

I looked for an equivalent way to sync Thunderbird too, but Thunderbird wouldn't let open an account in my Mac. It would not accept my password, although it was the same password that works smoothly in my Windows Thunderbird. Sounds like a simple but impossible problem --- like pushing your head through a stone wall.

You're welcome ! You can try to find what's going wrong with Thunderbird in the Thunderbird support forums (http://www.mozillamessaging.com/en-US/support/)