I’ve run into this same thing on all 47 WordPress sites I manage. For some reason, in Firefox only, pages and single posts have everything displayed as bold.
If I check the theme files and/or edit the page, there are no ‘strong’ tags, but if I use Web Developer to view the source there will sometimes be up to 6 nested levels of ‘strong’ tags. I have no idea where they come from, and this only occurs in Firefox.
View the same page in IE or Opera and it is fine. View source from IE and there are no strong tags. I never have figured out why this is, but I have found out from a couple of Church Site Design message boards that this is fairly common and not just me.
So we just assure our clients that for whatever reason Firefox and WordPress don’t play together nicely when pages are involved. Home page is fine, so if it is a blog site, it really doesn’t cause much trouble. If it is a whole church web site that we are using WP for, then we remind the client that although FF is generally the best browser, the majority of their site users will be using IE and will see the pages displayed as they are supposed to be.