Please, use mc (MidnightCommander), not PCManFM; with mc you can see symlinks.

Copy your Firefox-download from Mozilla site to '/opt', select the compressed file within mc and extract it with "F2 => z"

Change your view from the other panel of mc to '/usr/bin'

Move (=rename) '/usr/bin/firefox' to '/usr/bin/firefox-old' within mc with "F6"

Select in the first panel of mc '/opt/firefox-9.0.1/firefox'; make a symlink (with mc => File => Symlink) to '/usr/bin/firefox'

To get the icons is a little more tricky.

Change view in one panelside of mc to '/opt/firefox../firefox/chrome/icons/default', in the other panelside to '/usr/share/icons/hicolor/16x16/apps'.

Select the entry 'default16.png'

Create a "Symbolic link filename:" '/usr/share/icons/hicolor/16x16
/apps/firefox.png'

Do it in the same manner with 'default32.png' an 'default48.png'