Are you using FF 4? If so, did you go to: Tools > Add-ons? It'll open a new tab and show all of your add-ons/plugs-ins, etc.

Yes it's FF4 but as I said in my original question the 'Tools-Add-Ons" route merely shows the last updated add-ons - not a complete list. I know there are buttons to click for sorting by last updated and name but clicking on them achieves nothing.

They're all there, just divided into "Extensions," "Appearance," and "Plugins" instead of all being in one list. It sounds like you're opening the "recently updated" section and trying to sort the list from there. Try checking the other sections; if they're not showing up, that would be the problem. :/