Copy rectangle selection in firefox

If you hold down the CTRL key (or Cmd on a Mac), it lets you make that kind of selection. It doesn't paste into Excel how you'd expect, though, so you'll have to see if that's useful to you.

See the Firefox extension Table2Clipboard :

Mozilla applications allow to select rows and columns from a table simply pressing Control key and picking rows/columns with left mouse button.
The selection can be copied to clipboard but the original table disposition is lost making ugly results when you paste the text on datasheet applications (eg excel).
If you want to paste data in Microsoft Excel on OpenOffice Calc with correct disposition simply use Table2Clipboard.
Pasting in plain text editors is also supported as CSV file (but you can change rows and columns separators from option dialog)