Fortunately, as of this writing the vigilant Firefox crew have noticed and patched the bug in their most recent nightly release. First, export your bookmarks to a file (this worked for me using Camino, export to HTML). Then go to The Burning Edge (which tracks nightly Firefox builds) and download the official Mac branch build. Run it and import the bookmarks. Then, depending on how risk-averse you are, you can continue to use the nightly build, or you can quit, throw it out, and run the most recent official release instead. Either will recognize your imported bookmarks.
[robg adds: I just noticed this bug last week when trying to bring over some Safari bookmarks; nice to see it fixed. Running nightlies is typically not the most stable thing you can do; I would recommend using the install-run-import-revert method for most users.]

