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. There is no % option that appears, the Page or Text Zoom is done thru View > Zoom or from the keyboard. Firefox 4.0 will have increase/decrease buttons for the Toolbar.
https://support.mozilla.com/en-US/kb/Page+Zoom

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/

Your More system details list show that you have two Flash plugins installed.
One a very old Flash 5 version.

1. Shockwave Flash 5.0 r41
2. Shockwave Flash 10.0 r32

You should remove them both and update to a more recent version.

You can find the old Flash plugin file NPSWF32.DLL in the plugins folder within the Firefox program folder (C:\Program Files\Mozilla Firefox\plugins\) and remove that file.
Also remove the flashplayer.xpt file in the components folder.
See also:

• http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins

What do you mean with percent option?

Do you mean zooming a page?

• Easy Zoomer: https://addons.mozilla.org/firefox/addon/10216/
• NoSquint - https://addons.mozilla.org/firefox/addon/2592

See also https://addons.mozilla.org/en-US/firefox/addon/zoom-page/