Step 1: Download and install the plugin called "IE Tab". You can get it right from Mozilla's website.
Step 2: Go to Roblox.com
Step 3: Click on the little Firefox icon in the bottom right corner of the window. The ROBLOX website is now running under IE's engine, and can launch ROBLOX by visiting a place.
Step 4: To make it permanent, click Tools > IE Tab Options. Under URL, it should have http://www.roblox.com/ listed. Add an asterisk (*) to the end of the URL. Click Add. Now, ROBLOX will render in IE's engine by default.
(I got that right from builderman's help)