Two questions:

(1) Are you using one of the "standard" methods for cut/copy/paste, such as:

• Ctrl+x, Ctrl+c, Ctrl+v
• right-click context menu
• Edit menu on the classic menu bar

Or are you using buttons supplied by a website on its own editing toolbar?

(2) Does this affect all sites or only particular sites?

I use right click context. I just tested other sites - no issues, looks like it is just this site but what I am copying is a dropbox link which is very important- and long. On the site , it gives option for view in plain text which I choose then insert but it breaks it up in 2 lines - I am concerned that the link wont work properly. I appreciate your help, Thanks!

When you right-click the link, the page immediately tells you you can't copy, or does that occur after you choose Copy Link Location?

If it appears immediately, you can switch a setting in Firefox that should prevent sites from blocking the normal right-click menu. Here's how you would do that:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste context and pause while the list is filtered

(3) Double-click the dom.event.contextmenu.enabled preference to switch it from true to false.

This setting does have a complication: On Google Maps and other sites that have their own right-click menu, Firefox's menu then will block the site's menu. To clear Firefox's menu so you can access the site's menu tap either the Alt key or Esc key once (but sometimes Esc removes both).