Problems wuith font size in Firefox

I have not changed anything that I am aware of. Today I got up and the font
size on Yahoo's home page is small. If I click on a link it is the same as
always. If I go into preferencesm but change nothing, and then save
preferences, the font returns to normal for as long as I have Firfox open.
When I close it it reverts back to the small size page. Any suggestions?
Could it be Yahoo? My other links or bookmarks appear normal.

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