I am running FF (2) with Noscript and AdBlockplus, and I have no problem with content from ui.ibsrv.net.
Blocking it will prevent much of the site from rendering correctly.
You can block ibpxl.com without any loss of functionality.
From time to time I do notice that PPRuNe slows down, and this is usually with a "Reading...ui.ibsrv.net" message in the status bar. This is usually a few seconds, although it can be longer (and we mods do complain vociferously if it is bad).
I guess it is just busy, although Tues night / Wed. morning is backup time in IB land (West coast USA time), so a slow-down at that time could be related.
There's no conspiracy here, folks.
SD