When you refresh Firefox, all of your accumulated site-specific zoom levels, and any zoom-related extensions, are removed. So that would be my best guess as to why the text size looks different now.

Whether you are in private browsing or not in the same installation of Firefox should not make a difference. If that does make a difference, perhaps the Tracking Protection feature is affecting your blog's style sheets. If Firefox is blocking something on the site, a shield icon should appear toward the left end of the address bar. You can temporarily disable Tracking Protection on the page as described in this article to see whether that makes a difference: What happened to Tracking Protection?.

Oh, one other thing about private browsing is that regular windows and private windows do not share cookies, they have two separate "cookie jars." If your theme includes a text size preference saved in a cookie, that also might explain a difference between regular and private windows in the same session of Firefox.