I have a copy of a joomla-based website on my laptop, so I can mess around with it off-line, and I was getting frustrated because the font on my laptop copy was larger than the live version. I re-downloaded the css files, checked various config parameters and basically did everything I could to make sure my laptop had a true copy of the live version, and still, the fonts on my laptop copy were bigger than the live version. I even deleted my cookies.
As it turned out, it wasn’t the website, it was Firefox. I had, apparently, used Firefox’s zoom feature once to enlarge the font; and Firefox remembered, not just the page, but the server on which I had zoomed the font. In this case, the server was my local copy.
I hate when software does things for me without telling me. It’s so Microsoft.
Anyway, here’s an article about disabling this feature: http://www.mydigitallife.info/2008/07/03/disable-firefox-domain-sticky-zoom-to-reset-zoom-level-on-re-visit.