This is not a bug with the plugin, it is something in the actual javascript file as in you need to put an end limit to the distance the snow goes down.
If you leave it at default, it carries on falling forever & that is why your page keeps getting longer forever.
I put this on my site a few weeks ago when I did a Christmas skin & because of this, I had to change it.
With a forum, you cannot really specify the length as it changes on differnet pages, so I put it that it snows on the header image only...
So you can see it settling at the bottom of the header.
It also stopped the distraction of snow going across the topics & posts, so in the long run, I thought it was the best idea.
So, to stop it at a certain point, you need to add this in the script...
snowStorm.flakeBottom = 198; // stops at botton of header
Just change the number to where you want it to stop.