File Sharing Enabling Itself/ Possible Malware Infection?

Question

Greetings to all,
I'm posting this out of sheer desperation because no one else seems to have reported such an issue (or so Google would have me believe).
A bit of background info:
I'm running 10.8.5 and Firefox 24.0, no frills, no extra customisations apart from one add-on; pretty much everything left to the default settings (homepage, theme, etc.).
I noticed two weeks ago that my first tab in Firefox kept shivering every time a new tab was opened or an existing one was closed.  The only thing that had been installed was AdBlock Plus and that was running flawlessly for a month.
I researched the problem on my own and found that previous versions of Firefox had similar shaking issues but none were related to AdBlock.
I reset my browser and the shaking stopped.  I re-installed AdBlock and the issue returned in ten minutes.  I reported this to AdBlock Plus but have yet to hear back from them.
Now moving onto the crux of the issue:
I noticed in the last week that File Sharing has been turning itself on.  I have another Mac on the network but I have only used this feature a handful of times and always make sure to disable it as soon as I'm done.  This is combination with e-mails that I did not send that appear to be sent from my account despite having changed my password several times in the last year.  None of my contacts have reported that they received spam from me.
I had been mildly suspicious of some such malware due to Netflix/ Hulu magically playing videos on their own well after they had already timed out.
I checked Activity Monitor and there were no suspicious processes but I figured if a keylogger, for instance, were sophisticated enough, I still might not be able to see it.
Enter MacScan 2.9.4.  I downloaded it directly from their website and ran a full-system scan which ultimately yielded nothing.
I re-open Firefox and what do I see?  Yahoo! has been made my new homepage and a bunch of Spigot searchbars have been added, namely that for Amazon and eBay.
When I Googled the company name, almost all the of articles referred to it as malware.
I have since reset my browser and I'm starting to freak out more than just a little.
A well-known and recommended AV hijacks my browser, spam is being sent to me by 'me', File Sharing is enabling itself, and of course, the phantom video playing.
Has anyone seen this conglomeration of symptoms before and if so, what can I do about it?
If not, am I going to have to wipe both Macs?
Please some kind soul out there, help!

Answer 1

I'm not following your description well enough to know exactly what happened. What site were you viewing when this happened, and did it happen immediately on clicking a link or did it just happen spontaneously? What is a "corrupted install window"? And what is "the corrupt file" you found in your download folder? Without more information, it's difficult to say, but it doesn't sound like malware to me. Of course, if you wish to set your mind at ease, get a copy of [ClamXav|http://www.clamxav.com> and scan your hard drive. Also, I would point you to my [Mac Virus guide|http://www.reedcorner.net/thomas/guides/macvirus>, but as I've received private communications from you already, I see you've found it!
Note that files ending in .part are temp files... they are the beginnings of files that your browser started downloading. When you cancel a download, whatever had been downloaded to that point is left in a .part file, I believe to allow for resumption of the download later (if the server in question supports that). Whatever's in there is not complete and could not possibly be opened, so it is not a threat.