Ctrl-C, Ctrl-X and Ctrl-P should all work as shortcuts to copy/cut/paste like most Windows applications. You can also press the "Alt" key to see a regular menu with and "Edit" selections. Finally you can select text you want to copy with and right click to copy as well as right click in an empty entry area to paste.

You can find the Copy, Paste, and Cut buttons next to Firefox > Edit (and in its sub menu) and in the regular Edit menu on the menu bar (press F10 if the menu bar is hidden).
The Edit text is always grayed, but the three buttons next to it should become enabled if there is something to do like Copy if text is selected.
You can hover the arrow at the right to get a menu with Cut, Copy, and Paste.

You can find the Copy, Paste, and Cut toolbar buttons in the toolbar palette in the Customize window and drag them on a toolbar.

• https://support.mozilla.org/kb/How+to+customize+the+toolbar
• https://support.mozilla.org/kb/Back+and+forward+or+other+toolbar+items+are+missing

I found this works- copy by right click as usual or control +c then to paste is control + v