I've also tried running Firefox in safe mode, to no avail.

Try resetting Firefox to it's default settings.

https://support.mozilla.com/en-US/kb/Resetting%20preferences

If that doesn't work, it's possible that your company web site contains code that isn't compatible with Firefox. As you say, other browsers do not have an issue.

Try a browser compatibility testing web site, such as the one below (I have not used this site, but there should be others available)

http://browsershots.org/

Default settings didn't work, but the browser compatibility site is intriguing. I'll try that.

I've just tried browsershots.org with my own web site. It works quite well, but takes a little while to load all the screenshots.

Really, you only want to see if it works in Firefox.

You can also use an HTML validation tool such as below:

http://validator.w3.org/

It may point to any problems with the web site code.

Many thanks. The validator pointed to a few errors which I can try to fix; hopefully one of them was the cause. So far on the screenshots, all the browsers loaded the page successfully except Firefox 4.0+ for Ubuntu and Windows.

This is good to know. The validation tools are useful for seeing how every browser will display a particular web site.

Please mark my answer as the solution if you think it was useful.