Firefox 44 reduced the number of formats placed on the clipboard when copying text. While testing didn't show any ill effects, subsequent reports on this forum have identified Word 97, CorelDRAW, and Microsoft Works suite spreadsheet component as applications that cannot handle the more modern formats. So this will be changed back in Firefox 45.

Firefox 45 is scheduled to be released next week. Until then, the workaround continues to be pasting into an intermediate application, selecting the pasted content there, cutting, and then pasting into the old application. Unless someone comes up with something better in the meantime.