I needed a few Firefox extensions to be installed but Firefox just refused to add them. Refused? yes. When you try to install an extension, if the versions match, you should recieve a message saying the extension will be available when the browser is restarted. That message just did not appear. I tried downloading the xpi to the local hard disk and loading it from there with the same effect.
Then I tried updating firefox. No dice. Next I tried uninstalling and installing the whole browser again. Still no dice. Extensions just go missing. Then I had a brainwave and deleted extensions* in the .mozilla directory and restarted the browser. Eureka extensions are now appearing in the tools menu.
I have now re-installed the old ones and installed about a dozen new ones but more on them later.