hello, website zoom levels are part of the history information - so if you don't want this information gone when clearing you history make sure to un-check site preferences beforehand.

Delete browsing, search and download history on Firefox

You can modify the layout.css.devPixelsPerPx and increase or decrease the value in 0.1 or 0.05 steps to adjust the size of fonts and other elements in Firefox.

• The layout.css.devPixelsPerPx pref is a String value parsed to a float and allows to fine tune the dimensions of all elements (user interface and web pages) more precisely (resolution 0.1 or 0.05).
• You can use values between 0.05 and 1.0 to reduce elements in size or use values greater than 1.0 to magnify and make elements larger (percentage divided by DPI, % / DPI).

If the web pages needs to be adjusted after changing this pref then you can look at Default FullZoom Level or NoSquint.

• Default FullZoom Level - https://addons.mozilla.org/firefox/addon/6965
• NoSquint - https://addons.mozilla.org/firefox/addon/2592

See also this MozillaZine forum thread.

Does Fx 22 nightly look too large?:

• http://forums.mozillazine.org/viewtopic.php?f=23&t=2678897