The best way I found to copy in Firefox or Internet Explore. Highlight the text or picture, then select the text or picure you wish to copy. After you select the text then mouse over the selected texted then right click with the mouse button, a pop up menu will pop up. Click on "copy", then move the mouse to where you would like to paste it, then do the same there by right clicking the mouse button, pop menu will show "paste". It this does not work then the copy on the web document or web page has been turned off so no one can copy the text.
Firefox does have alot of add-ons you might find something in the add-ons list that will allow you to copy an item. If the web site or web texted has been set to not allow copy of this document or text then there is nothing out the that will allow you to copy.
You can also use the keyboard like Mabel says, or you can also use the pull down menus at the top of Firefox.