Firefox often does this while cleaning up after itself and one of the processes responsible is named plugin-container.exe which, depending on the exact plug-ins you have installed and enabled can take a few minutes.
If it takes more than 3-6 minutes and other processes aren't using the CPU to any extent you might try killing firefox.ece and plugin-continer.exe manually via your TaskManager.
Dell Inspiron 1545
I am experiencing difficulties when re-opening F'fox. You close it on the shortcut, that works fine, but, when you want to re-open it it informs you that it is still running, but it isn't, or there's nothing to indicate it is. The delay can be a good few seconds at best, with up to a couple of minutes. Sometimes I have re-start the pc.
I've reinstalled to no avail. Any advice please.