Help - does anyone know how

I thought I found a solution but looks like it won't work.  Sorry.

I don't think you can install IE or Firefox but you could try a user agent switcher.  that will make the website think you are using IE or Firefox but it doesn't guarantee that it will render properly.

https://chrome.google.com/webstore/search/user%20agent%20switcher

I know this is a couple days old, but you can't really install anything other than chrome addons to a chromebook.  The Chrome OS is really nothing more than the chrome browser and that's it, so you're limited to things you can do inside of chrome only.

You might be thinking of me, I work for Microsoft actually on the IE team.

The beauty and curse of a Chromebook is that it runs ChromeOS which is basically a souped-up Chrome browser.  It can't run or install anything not specifically built for it.  No Mac, Android, or Windows software.  That makes a Chromebook dead simple, highly reliable, really cheap, and very limited. 

The user agent switcher extension may work, if the internal message boards are doing some simple stuff to block non-IE browsers.  But even if that appears to work, it's likely the website is written targeting older IEs like IE8 from the WIn7 era, and Chrome isn't all that interoperable with IE8.  By "interoperable" I mean Chrome targeted websites don't work well in IE8, and IE8-targeted websites don't work well in Chrome / Chromebooks.  So things might appear to work at first, but then have problems.