I use Firefox for preference but it doesn't support FB. My machine is about two years older than Facebook itself. I have tried both Opera and SeaMonkey but they were too flaky to put up with. I use no security software whatever, beyond what the router's firewall provides. If it was a security/filtering issue I'd be seeing it with other websites - only FB displays this phenomenon.

There has to be something different about the way FB displays plain text.

BTW I have encountered something analogous under Windows XP Pro, where copy/paste between websites in different tabs was sometimes blocked (not consistently). That probably was a security issue - the machine belonged to my employer, whose IT department are both paranoid and stupid.