The Add-ons page only shows a Remove button for add-ons installed into your currently active Firefox profile folder. Firefox finds most plugins outside of that folder through a Windows registry key where third party installers advertise them, or in a shared folder.

To find the physical locations of plugins on disk, you can use the following internal page: type or paste about:plugins in the address bar and press Enter to load it. Each plugin's information will include the full path to a file that you can remove if you like. I suspect that won't break any other software.

Krull said

I have a load of old plugins in Firefox that will not die I do not want these showing as I do not use them Under plugins tab of add-on manager they have three options Ask to Activate Always Activate Never Activate but there is NO REMOVE option

There is no Remove for Plugins in Firefox on Windows, Mac OSX, and Linux simply because unlike Extensions and Themes (complete), the Plugins are installed on system and not in Firefox. The Activate settings just tells Firefox how to make use of that compatible Plugin it found on system.