The Firefox Page Zoom feature does a domain by domain level of saving the users preferred zoom level settings, there is no default Page Zoom level setting in Firefox.

Try the Default FullZoom Level extension:
https://addons.mozilla.org/en-US/firefox/addon/6965

Or the NoSquint extension:
https://addons.mozilla.org/en-US/firefox/addon/2592/

Thanks

I have just upgraded to 5.0. Whenever I start up Firefox the web page is too wide and I can only see half of the page in the window. As soon as I start to adjust the window size it corrects itself and I can see the whole web-page. This was never a problem in 4.

If you got Windows 7, click start, control panel, display (make sure it's set to smaller (default), go to upper left click adjust resolution, advance settings, list all modes and then move up to the next higher resolution that's set for your computer. This solved my problem.