Firefox remembers zoom settings for every individual website you open. An example: if you open
www.sevenforums.com and zoom that to 200%, and then open
www.cnn.com and zoom that to 150%, these zoom levels are remembered. If you close Firefox and reopen it, next time you go to
www.cnn.com it is going to be opened zoomed to 150%, and
www.sevenforums.com will open zoomed to 200%. If you do not reset the zoom before opening a new website or closing the Firefox, the zoom level is always remembered.
You can test this easily: open a few tabs and a separate website on every tab. Set then different zoom levels for every tab. Now close Firefox and reopen it, opening the same pages and you'll notice every page remembers the zoom level.
Back to your question: could it be that though you want to start your Firefox homepage normally, you maybe later zoom it in and forget to reset zoom before closing the page or Firefox? As far as I know, this is the only way to get Firefox open a website so that it is already zoomed in.
Kari