Fire Fox is changing page size

Hi eveyone,
I’m having a pet peeve with my word press twenty eleven theme in Fire Fox on my laptop only.

When I enter my site on my landing page, the display is a smaller size. (this is the size I really like.)

But, when I go from my entry page to another on the website, the page seems to super size itself and then it remains in this size.

Again, this is only happening on my laptop, an acer aspire, and not on any other computer in the house. (we’ve 6 others).

Any help on a fix would be greatly appreciated.

The site is: https://www.bengalcatsaustralia.com