See "Hang at exit":

• http://kb.mozillazine.org/Firefox_hangs
• https://support.mozilla.com/kb/Firefox+hangs

Thanks to the answerers; I'd have gotten closer by myself if there had been some ëarlier nexus between my "doesn't close" and the knowledgeable community's "hang". Now my task is to re-enable the add-ons one by one to isolate the culprit. So far the Orange Surge" theme has been exonerated.

Seems as if it is Symantec IPS 3.2 that has the effect of blocking firefox.exe's closure.

I do not have an answer myself but I have seen this issue for some time now. I recall during the installation of the latest version of Firefox that it complained about Symantec software and some kind of incompatibility but I don't recall what it was. I do recall it saying that it would disable the feature but when I went into the addons (this morning) and looked they were all enabled. I narrowed down the hang to the exact problem everyone has above. I am questioning what happened between the newest install and finding the offending addon enabled? I don't know how that would have happened. Did a new Symantec install add in a newer version and re-enable it? Did the Mozilla install fail to disable it in the first place? Does this issue really belong to Symantec?