Several webpages have a copy protection, ie. you cannot copy texts and / or objects by marking a section, then copy it by pressing the Right Mouse Button. Which can be very inconvenient, for example if you try to comment on something somebody else posted.
In FireFox that's pretty easy to fix. (Though it doesn't solve everything.)
Go to about:config and change dom.event.contextmenu.enabled to from True to False and most websites can no longer block a right click.
Unfortunately it only works for some websites. There are still many left where you simply cannot copy the text this way either.