The sentence is saying that there was a bug in a previous version of TenFourFox that caused the browser to hang when browsing to Facebook, and they speculate that they have fixed the bug in this release.
ah sorry about that, the cookie warning was folded over the subtitle and missed that line; wrongly assumed it was an operating system.
I suspect it's as it reads. This blog appears to be about a browser codebase/product. I'm guessing a report that "it hangs on Facebook.com".