I thought the Bookmarks Toolbar always took the bottom position, but I haven't installed many extensions that create toolbars, so perhaps it's not a strict rule.

Toolbar layout is stored in an XML-format file named localstore.rdf. It is a temperamental file that sometimes becomes corrupted and is tricky to edit. However, editing or deleting the file is far less disruptive than a full reset so, for future reference, you can find it here:

Open your current Firefox settings (AKA Firefox profile) folder using

Help > Troubleshooting Information > "Show Folder" button

Leaving that window open, switch back to Firefox and Exit

Then pause while Firefox finishes its cleanup before editing/deleting localstore.rdf (use a plain text editor to view/edit)

That fixed it, thanks