Printing the layout helps, I can pretty much do it from memory..

Once you have freshly generated the layout you want

1. Open your profile folder by typing about:support int the location bar (or Help > Troubleshooting information...)
2. Click on Open containing folder
3. use File > Exit or "Firefox" button > Exit to get out of Firefox (Not the "X" in upper right corner as that just closes the Window)
4. in your profile f older select localstore.rdf and copy ("Ctrl+C") then paste it aback to make a copy, repeat to create at least one more copy, The original and "localstore - Copy.rdf " and "localstore - Copy (2).rdf" will all be the same, about 4KB to start with. (not 18 KB not 76 KB)
5. bring Firefox back up.

Freshly generated means rename or delete the original localstore.rdf while Firefox is down, restart Firefox and customize your toolbars as you want them.

If you have a problem with a corrupt localstore.rdf bring Firefox down and in your profile directory delete or rename the localstore.rd then rename a copy to localstore.rdf

Reference: http://kb.mozillazine.org/Corrupt_localstore.rdf see references to backup copy, you should, of course, be taking periodic backups of all your personal data, and for Firefox that includes your profile.

dmcritchie - you're a bloody genius - thanks I've been hunting the internet trying to find this info for ages!