There's two extensions that allow you to set a default zoom level.

Default FullZoom Level extension - I had problems with it on Ubuntu 10.4 -
https://addons.mozilla.org/en-US/firefox/addon/6965

NoSquint - didn't try it on Ubuntu 10.4 -
https://addons.mozilla.org/firefox/2592/

thanks for your help,still not working

Are you running Firefox in permanent Private Browsing - Use Firefox without saving history mode ?

• You enter Private Browsing mode if you select: Edit > Preferences > Privacy > History: Firefox will: "Never Remember History"
• To see all History settings, choose: Edit > Preferences > Privacy, choose the setting Firefox will: Use custom settings for history
• Uncheck: [ ] "Automatically start Firefox in a private browsing session"

the-edmeis, many thanks for your help,it worked a treat,sorry ive being so busy getting to know this new os, linux mint 9,but im getting there slowley thanks again