My original post in the "Am I infected? What do I do?" forum can be found here.
(Note that I tried to post the FRST log below as instructed, but was told: "Your post was too long. Please go back and shorten it a little." I guess it was exceptionally long for some reason? As such, I have included BOTH the FRST log and Addition.txt as attachments)
All details, screenshots etc from my problem are given at the above link, but in summary, my laptop (running Firefox 40.0.3 under Windows 8.1 64-bit) and my father's desktop (also running Firefox 40.0.3 under Windows 8.1 64-bit, and using the same router) are receiving 301 and 302 errors when trying to follow certain links (including one posted by BleepingComputer helper Broni) in certain ways.
As described (and screenshotted) at the link above, alternate left-clicks on such links consistently provide different results (but neither gives the desired target page). Even-numbered clicks give "about:blank", and odd-numbered clicks give the "301 Moved Permanently" screenshot given in the above forum post. These links work fine if I right-click and "open in new tab", or if I copy them to my bookmarks toolbar and then open them through that.
Clearing the cache does not affect this behaviour, and exactly the same symptoms apply to a number of html links that I have encountered (e.g. the three given in the profile of this Twitter user). My best hypothesis about the commonality between the links to which this applies is that perhaps they are all multi-stage redirects.
Another laptop (my old one - running Firefox 40.0.3 under Windows 8) is unaffected and functioning normally, despite using the same router/connection.
These problems may or may not be related to disruption to our internet service (which became slow and intermittent when this issue began, though our ISP has since resolved this and it is now functioning well) and the fact that RSS feeds are intermittently resetting to 18th August (i.e. only displaying posts up to 18th August, rather than the most recent ones).
Guided by Broni at the above forum I have run a number of scans. I also resolved the internet connectivity issue with my ISP in case this was the cause. However, symptoms are persisting (and only on certain computers), which makes me suspect malware infection. As you can see, after taking me through several investigations Broni has asked me to run FRST and post the logs here, which I have done - find attached.
Any advice much appreciated, thank you.