I'm getting the same thing! I thought I was going crazy. It happens for me only once in about fifteen or twenty emails, though.

I also use Firefox, but I have an Intel-managed computer that's pretty much guaranteed to be free of any sort of malware. It happens only on this site, and only when I click on a thread notification I received by email. It never happens when I click on mognet notifications or anything else on this site, or on any other. I can't even click 'back' to return to the thread.

What's odd is that it seems to at least momentarily access eoff - long enough to register me as having accessed the thread for "new post" purposes. To fix this problem, I double-click on a thread notification, and if one gets redirected at least the other doesn't!

This is almost definitely happening server-side.