Firstly , the problem you are describing has nothing to relate to the Serial number.
Secondly , you wrote that Safari and Chrome does work fine , hence sounds more like Firefox related issue.
Can you check do you have PDF plugin enabled in the firefox?
Also , uninstalling and re-installing the firefox , does that help?
Understand, again, that the ONLY difference was registering the product by entering the serial number. When the "trial" version was installed, PDFs opened in Firefox fine.
I would agree (with some of the other responses) that Firefox has become more problematic in recent times, and I too can trace many of the problems to the development team's notion that rapidly increasing versions are a Good Thing.) They are not, BTW, an even annual releases of products as large as Adobe products are also not good.) But Firefox also has great development tool add-ons, so as a default is not bad. Suggesting a switch to another browser, while potentially satisfying (especially if you are Googler or Google fanboy) doesnt actually identify or solve the problem. And no response so far has actually attempted to do that.
None of the persons responding to your post are Googlers or Google fanboys. Manish is an Adobe technical support engineer and both Steve Werner and Bob Levine are exceptionally competent and knowledgeable contributors to these forums.
It is indeed true that we at Adobe and other software developers have had significant problems with rapid updates of FireFox that have automatically disabled what the FireFox developers believe to be incompatible plug-ins. FireFox updates (either explicit or without your knowledge) may or may not the cause of the problem you see.
What we can tell you is that as far as we know at Adobe about the code and based on the lack of similar reports from others with such a symptom, there is absolutely nothing in the serialization / activation / registering of the Adobe Creative Suite software that would muck around in any way with the Adobe PDF plug-in for Safari or the plumbing used therein.
Manish asked whether you checked as to whether the PDF plug-in to FireFox is enabled. Please check on that. If it was somehow disabled, try re-enabling it and see if that resolves the problem after restarting FireFox. If the PDF plug-in was already enabled, re-enabling the plug-in doesn't resolve the problem, or you don't see the PDF plug-in option at all, please advise.
I have just had this happen, and I'm pretty sure it's since I installed CS6. I did some googling and someone said to disable the Adobe Acrobat NPAPI plugin. I looked in my addons and I didn't have it. I don't think I'd restarted Firefox since installing CS6, so I quit and restarted. Still no change: going to a PDF download would bring up a blank window with the PDF file name and no download activity.
Went into addons again, and there was the plugin this time: the restart somehow made it show up. Perhaps it was installed by CS6? Maybe I just missed it the first time around. Anyway, disabled it, and tried again, and my PDF downloaded properly.
I'm not sure what the NPAPI plugin is supposed to do, but I'm leaving it disabled until I have a reason to turn it on again.