fix "Paste text in firefox add <br> and <p>"

For some reasons, Firefox doesn't paste correctly. When I try to paste a text, Firefox adds a `br` and a `p`, then paste the text inside the new `p`. On the other hand, Chrome is working correctly, It adds in same existing `p` unless the string was a multi-line, it creates a `p` for each new line.

So I tried to fix Firefox behavior to be the same as Chrome's