I have tries copying content from google docs spreadsheet using firefox 37, chrome 41 & IE 10 it works in all three browsers.
I understand that due to security restrictions browsers do not allow access to clipboard (specifically firefox) then how am I able to copy from google docs spread sheet running in browser to excel spreadsheet running on my windows machine?
Does this involve using flash plugin?
This does not use flash. Actually, you can access the clipboard, but this isn't what is happening here. Google sheets uses a <canvas> element to display the sheet, so when you select a cell you are actually selecting an invisible html table that contains the selected data. When you copy the data, you copy the html table into your clipboard. When you paste into excel, excel recognizes the table element and converts it to an excel spreadsheet.
You can actually access the clipboard in all modern browsers (chrome, safari, firefox) when information is pasted. For example, this demo.
