You May have already done all this, but just in case others may be experiencing...
Just as a first step i would remove the MOZBAR (Not disable actually remove fully)
Second clear all cache and cookies (clearing all your PC's temp and prefetch can sometimes assist as well, but not necessarily relevant. And yes i know there are many articles out there that say do not clear prefetch, but they are incorrect... 
Next reinstall the Addon
Just from years of support, this is the first steps I would take on any addon no matter the author. Add ons source code is rarely the issue, and most often system specific. But of course there are always exceptions to the rule!! 
W00t
Realized after making blanket statement "Add ons source code is rarely the issue, and most often system specific" ..... That could be misinterpreted. Speaking specifically to hung processes, this is caused by a DLL interacting incorrectly, and if your temp and cache is full, this adds to the issue especially in this instance because the browser itself begins reacting slower, because it is having to shift around data every time a query is made. In the instance of Mozbar, I would assume it makes a "call" on almost every interaction with browser. This is the reason for both the Temp and cache clear as well as reinstall. (Just to verify it gets a "fresh start" at interacting with system resources)
But of course this could be the simple fact that the addon is making so many calls.... Hope i cleared something up (probably not LOL)