[SOLVED] Browser compatibility

I'm having a problem when I view my site in Firefox that I do not see when I view the site in Chrome.

My screen size changes depending on where I am on the site as I’d like it to stay all the same size.

On first visiting the site it looks like this:
Clicking elsewhere on the site, even clicking on the logo, makes site “wider”
Clicking onto logo to return to home page, site stays “wide” .e.g. see how scroll on featured has disappeared due to the wider width.
I'm using 1.5.4 and Shopcart theme, a link to my test site is here http://www.braeness.co.uk/newstore

Any help greatly appreciated! Thank you, Melanie

I've worked it out - the problem was with my config.php files - adding www to the site address made the problem go away.

Thanks, Melanie