Odd behavior with right clicking

[tim]

Binary package hint: firefox-3.0

Go to any website that has multiple images on it.
Place the mouse pointer on top of an image and right-click.
Select "Save Image As... " and save the image.

After doing the above 3 steps for anywhere from 3 to 10 images (seems to be random as to how many you have to perform this for before the problem appears) then, from time to time and very frequently, instead of the pop-up menu appearing from which you can select "Save Image As..." firefox simply opens up a new window as if you'd selected "Open Link in New Window".

I am running Ubuntu 8.04
Firefox 3 Beta 5 (3.0~b5+nobinonly-0ubuntu3)

ProblemType: Bug
Architecture: i386
Date: Sun May 18 03:00:04 2008
DistroRelease: Ubuntu 8.04
Package: firefox-3.0 3.0~b5+nobinonly-0ubuntu3
PackageArchitecture: i386
ProcEnviron:
 PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: firefox-3.0
Uname: Linux 2.6.24-16-rt i686