Create a new profile as a test to check if your current profile is causing the problems.
See Basic Troubleshooting: Make a new profile

If that new profile works then you can transfer some files from the old profile to that new profile (be careful not to copy corrupted files)

See http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

I tried both the instructions to type into Win 7 Start - neither worked. Think the best thing might be to uninstall and reinstall Firefox. Not sure what could have happened as I have exactly the same set-up on my laptop, also runing on Win 7, though not 64-bit.

Reinstalled, nav bar back, bookmarks back (thanks to the wonderful Xmarks - thank heavens they didn't go under.). I assume that I will have to re-enter passwords, etc. When I have more time I will look at transferring some of the old files to cope with this. For now just happy to have my nav bar back. Thank you so much for your help.