A web site is able to check what browser you claim to be using.
What is my user agent?
http://whatsmyuseragent.com/
In the olden days some websites got this check wrong. The work-around was to change your user agent to something the site supported like IE.
Look for an Addon the allows you to change your claimed user agent.
There are two techniques for checking browser capability
1) The simplistic but not recommended way is to check on what your agent you claim to be using.
2) Check the capabilities of whatever your browser supports. If the web site needs to use css, it checks whether or not the browser supports css. This is the perferred way of doing things.
Robert