There are several things that could be happening and they range from annoying to serious infection.
1) Double-Clicking. Vbuzzer looks like a type of company that may advertise on this site. Its possible that this site was loading and you clicked some where on the page before it was done and went to the Vbuzzer site. This is not likely as you said its happened several times
2)Local Cache problem. Website are often cached to speed up browsing, the address reside in cache and its possible that you have a bad address there. You can clear the cache on Firefox by going to Tools, Options, Privacy tab then clicking on Clear Now. You can deselect everything except Cache (or select everything except Saved Passwords if you want)
3) Local Install issue. I looked around and I dont see lots of complaints about vBuzzer spyware or hijacks but there are several VBuzzer apps that can be installed such as a messenger. you may have one installed on one of the apps settings is to set your page to re-direct to VBuzzer. This is not a problem, just annoying.
4) Try checking your Homepage. Tools, Options, Main, Home Page. Many apps change this for you without you knowing. If you reset it and it comes back you have an issue
There are lots of other causes but these are a few. Also the fact its so random makes it hard to guess. Most of the time if its something above it happens all of the time vs just at random time