As Firefox users are probably aware, not all sites work 100% correctly in Firefox. You can typically get around this in a few ways, so here is another method to add to your toolbox.
The User Agent Switcher Firefox add-on allows you change the way Firefox reports itself to a website. For example, you can set the User Agent Switcher to make Firefox report itself to websites as IE. This way the web page will send information to your browser as though it were IE and not Firefox.
For the most part, this will allow you to easily get around Javascript checks or see how your “IE optimized” HTML is rendered in Firefox. Keep in mind though, this add-on only reports Firefox as a different browser and will still render the page using the native Firefox engine.
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You can also use the ietab extension. It will allow you to run IE from a tab in firefox.
https://addons.mozilla.org/nl/firefox/addon/1419
You know, I bet people would pay greatly for an IE plugin that turns IE in to Firefox. Firefox may not be perfect but it’s hell better than IE.