Happened on this in an old Walt Mossberg column that I had saved. He gets all the credit. Worth knowing:

To back up or save a file with all of your bookmarks in Firefox (latest version works OK):

Select "Organize bookmarks" from the Bookmarks menu. With that window open, go to the FF File menu, select "Export." The Format choices (bottom of the window) are "All files" or "HTML files." I tried both and there didn't seem to be any difference in the results.

This saves a browser file with ALL of your saved bookmarks, so it may be lengthy.

Didn't try this, but according to Mossberg's comments, this file can then be imported into FF, again with the "Organize Bookmarks" window open, and using "Import" from the File menu. Thus you could copy them to a different computer browser.
- Don't know if it would work with Safari, if you wanted to transfer bookmarks to another browser, but worth a try.

Rather than import/export. I use a program called FoxMarks. They renamed it to get other browers to adopt it. Xmarks now??

Essentially this service logs all of your bookmarks from computer to computer. It syncs them across every computer you use.