strange jerky navigation with firefox

Is anybody have this strange behavior with firefox and cumulus together.
I have cumuls block on left side on every page on my site. If I use firefox every time I move from one page to another the change is little jerky. If I disable the cumulus block the navigation become again smooth. I don't see this strange behavior with Internet Explorer.
You can see same behavior on cumulus's "Try out demonstration" site every time you go from any page to the "tags" page there is a jerk....
am I the only one that can see it ?