Issue 4854: XHR breakpoints can't be removed from Breakpoints side panel

Steps to reproduce

  1. Open Firebug
  2. Enable the Script and the Net panel and switch to the Net panel
  3. Reload the page
  4. Click the Make XMLHttpRequest button
  5. above
    ⇒ An XHR will be made and listed inside the Net panel
  6. Click the breakpoint column besides the XHR entry to set a breakpoint
  7. Switch to the Script panel and there to the Breakpoints side panel
    ⇒ The XHR breakpoint is listed there
  8. Click the X on the right side to remove the breakpoint

Expected result