Can't drag launcher icons above or below first/last ones

I can confirm this.

*Note*
You must be dragging the icon inside the launcher. If you drag the icon outside the launcher then you place it correctly.

I can confirm this as well using any launcher icon and not just firefox.

This bug was fixed in the package unity - 6.4.0-0ubuntu1

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unity (6.4.0-0ubuntu1) quantal-proposed; urgency=low

  [ Łukasz 'sil2100' Zemczak ]
  * New upstream release.
    - Software Center add to launcher icon animation is delayed when the
      launcher is in auto-hide mode (LP: #1012896)
    - Home lens: Applications show up in Music category (LP: #1040101)
    - WARN unity.hud.hud Hud.cpp:185 Attempted to close the hud connection
      without starting it (LP: #1039451)
    - Text field textures do not update when maximum width changes.
      (LP: #1037187)
    - An application is closed if manually removed from favorites on gsettings
      (LP: #1032157)
    - The switcher (Alt+Tab) is showing incorrect windows after using Deskop
      Wall (Ctrl+alt+<direction>) (LP: #1031959)
    - Dragging icons to reorder -away from launcher causes the dragged icon
      image edges to fade away(cut off) (LP: #1026247)
    - Screen flickers when changing launcher icon size. (LP: #1024113)
    - Arrow for indicating lenses points empty space on alt+F2 (LP: #998752)
    - [previews] Music album tracks do not have a scroll-bar when needed.
      (LP: #1039462)
    - [USE_MODERN_COMPIZ_GL] With unity scrollbar support enabled, hotplugging
      monitors doesn't work (LP: #1039410)
    - [previews] Track titles aren't escaped (LP: #1039020)
    - Launcher - Edge of tiles (with backlight disabled) is too opaque
      (LP: #1038138)
    - [USE_MODERN_COMPIZ_GL] Benchmark window appears behind the Unity shell
      now (LP: #1036609)
    - [GLES] [USE_MODERN_COMPIZ_GL] Unity shell (Launcher and Panel) are
      missing if framebuffer_object is disabled (LP: #1036569)
    - [USE_MODERN_COMPIZ_GL] Benchmark window is mostly invisible, shrunken
      and displaced (LP: #1036528)
    - Unity quicklist menu items leak memory (LP: #1036338)
    - Icon resized between 32 - 48 - 64 look jaggy (LP: #1036231)
    - [Regression] Hideous low-res icon when using the HUD with autohide
      enabled. (LP: #1035951)
    - [regression] Selecting an item of the BFB quicklist when the dash is
      open, causes it to hide (LP: #1035641)
    - Launcher - Switch on "backlight toggles" mode by default (LP: #1033866)
    - Unity spends 15% of time in PlacesGroup::ComputeContentSize when opening
      dash (LP: #1033306)
    - Launcher dragged icon is not redrawn when the mouse pointer is not moved
      (LP: #1032700)
    - Bottom - Up key navigation is broken in dash filterbar (LP: #1028810)
    - Launcher - SD card can not be ejected by drag-n-dropping to trash
      (LP: #960910)
    - Alt-tab - The app name in the integrated menu should not change during
      Alt-tab switching (LP: #950782)
    - Dash - If only one Category Header is displayed, it should be
      automatically expanded (LP: #950710)
    - Launcher - Unity mouse wheel scroll too small (LP: #906072)
    - boot moves desktop icons (LP: #886667)
    - Alt-tab - Reduce the spread delay for all items other than the initial
      item the select lands on after alt-tab opens (LP: #838232)
    - Launcher - Launcher should never autohide when the cursor is positioned
      over the Launcher (e.g. When ...

Update: Unity 6.4 is revision 2632, so this is actually fix committed for Unity 6.6.