The standard advice is to develop in Firefox, and then fix any problems that show up in IE.
If you develop your site according to web standards, it will display correctly in Firefox and most other modern browsers. IE bugs/quirks are mostly known and workarounds are readily available.
Developing for IE first means you are painting yourself into a corner, and may have bad code that will require major head-scratching to untangle for standards-compliant browsers.
It is true that IE has the lion's share of the market due to Microsoft being the operating system builder for most pc's. I read recently that IE6 and IE7 each currently have around 1/3 of the usage, with Firefox around 1/4 and others dividing the leftovers. This can vary greatly depending on the target market.