Nope, It is a common issues with Firefox although it has been greatly reduced in the last year or so.

The project to fix all this is called e10s. Moving Content, and UI etc to seperate process as well as OMTx ( Off Main Thread Everything ) where Firefox Dev trys to do everything in async style and move them off Main Thread.

Progress has been slow. But at least they are working on it

It's a core problem with FF. Everything runs under the same process so a single site with slow / frozen JS will affect the whole browser.

matthiasv, I don't know what your secret is, only 1 tab open in 1 window open at time, JS disabled, or what, but this is a very real issue. I have Firefox installed on all 4 of my machines, work desktop, home desktop, laptop, and a netbook. 2 of which are fresh installs. On all 4 when I visit multiple HTML5 heavy sites, FF's UI gets unresponsive. Chrome and newer versions of IE do not suffer from this, and I'm pretty sure it comes from FF running in a single process.

Chrome and IE have their issues too, like thrashing memory a lot sooner than FF, which is why FF is still the default browser on my laptop and netbook despite its unresponsive UI under heavy load.